Support selecting microphone by name

Normally Mycroft will use the default PortAudio input device as the
microphone input for the user.  However in some cases there is reason
to specify a different input.

The "device_index" under the "listener" section in mycroft.conf has
always allowed a user to select the microphone explicitly.  But on
some systems the indices can change from reboot to reboot.  So this
adds the "device_name" setting.  If it exists (and the "device_index"
has not been specified explicitly), a regex match will be run against
the PortAudio device names.

When "device_name" is used, the voice.log will contain a listing of
the devices as they are tested.  This can be used to debug if a
name isn't matching as expected.

EXAMPLES:
/etc/mycroft/mycroft.conf
```
{
    "listener": {
        "device_name": "aawsrc"
    }
}
```
Would find a match against "aawsrc" or "aawsrcplug".  To force a specific
match, you can use a regex such as "aawsrc$".

/etc/mycroft/mycroft.conf
```
{
    "listener": {
        "device_index": 2
    }
}
```
The PortAudio device index specified will be used.

Names and indexes for PortAudio are difficult to guess.  The simplest way to
view them is either enter a value for "device_name" and look at the names
which appear in the log when starting Mycroft, or to run a simple program
such as:
```python
import pyaudio

pa = pyaudio.PyAudio()
for i in range(pa.get_device_count()):
    dev = pa.get_device_info_by_index(i)
    print((i, dev['name'], dev['maxInputChannels']))
```

mycroft/client/speech/listener.py

@@ -16,6 +16,8 @@ import time
 from threading import Thread
 import sys
 import speech_recognition as sr
+import pyaudio
+import re
 from pyee import EventEmitter
 from requests import RequestException, HTTPError
 from requests.exceptions import ConnectionError
@@ -225,11 +227,29 @@ class RecognizerLoop(EventEmitter):
         self.lang = config.get('lang')
         self.config = config.get('listener')
         rate = self.config.get('sample_rate')
+
         device_index = self.config.get('device_index')
+        if not device_index:
+            device_name = self.config.get('device_name')
+            if device_name:
+                # Search for an input matching given name/pattern
+                LOG.info('Searching for input device: '+str(device_name))
+                LOG.debug('Devices: ')
+                pa = pyaudio.PyAudio()
+                pattern = re.compile(device_name)
+                for i in range(pa.get_device_count()):
+                    dev = pa.get_device_info_by_index(i)
+                    LOG.debug('   '+str(dev['name']))
+                    if dev['maxInputChannels'] and pattern.match(dev['name']):
+                        LOG.debug('    ^-- matched')
+                        device_index = i
+                        break
+        LOG.debug('Using microphone (None = default): '+str(device_index))
 
         self.microphone = MutableMicrophone(device_index, rate,
                                             mute=self.mute_calls > 0)
-        # FIXME - channels are not been used
+        # TODO:19.02 - channels are not been used, remove from mycroft.conf
+        #              and from code.
         self.microphone.CHANNELS = self.config.get('channels')
         self.wakeword_recognizer = self.create_wake_word_recognizer()
         # TODO - localization

mycroft/configuration/mycroft.conf

@@ -158,6 +158,12 @@
       "url": "https://training.mycroft.ai/precise/upload"
     },
 
+    // Override as SYSTEM or USER to select a specific microphone input instead of
+    // the PortAudio default input.
+    //   "device_name": "somename",  // can be regex pattern or substring
+    //       or
+    //   "device_index": 12,
+
     // Stop listing to the microphone during playback to prevent accidental triggering
     // This is enabled by default, but instances with good microphone noise cancellation
     // can disable this to listen all the time, allowing 'barge in' functionality.