Propose adding feature flags to velero

Signed-off-by: Nolan Brubaker <brubakern@vmware.com>

design/feature-flags.md

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+# Feature Flags
+
+Status: Accepted
+
+Some features may take a while to get fully implemented, and we don't necessarily want to have long-lived feature branches
+A simple feature flag implementation allows code to be merged into master, but not used unless a flag is set.
+
+## Goals
+
+- Allow unfinished features to be present in Velero releases, but only enabled when the associated flag is set.
+
+## Non Goals
+
+- A robust feature flag library.
+
+## Background
+
+When considering the [CSI integration work](https://github.com/heptio/velero/pull/1661), the timelines involved presented a problem in balancing a release and longer-running feature work.
+A simple implementation of feature flags can help protect unfinished code while allowing the rest of the changes to ship.
+
+## High-Level Design
+
+A new command line flag, `--features` will be added to the root `velero` command.
+
+`--features` will accept a comma-separated list of features, such as `--features EnableCSI,Replication`.
+Each feature listed will correspond to a key in a map in `pkg/features/features.go` defining whether a feature should be enabled.
+
+Any code implementing the feature would then import the map and look up the key's value.
+
+For the Velero client, a `features` key can be added to the `config.json` file for more convenient client invocations.
+
+## Detailed Design
+
+A new `features` package will be introduced with these basic structs:
+
+```go
+type FeatureFlagSet struct {
+    flags map[string]bool
+}
+
+type Flags interface {
+    // Enabled reports whether or not the specified flag is found.
+    Enabled(name string) bool
+
+    // Enable adds the specified flags to the list of enabled flags.
+    Enable(names ...string)
+
+    // All returns all enabled features
+    All() []string
+}
+
+// NewFeatureFlagSet initializes and populates a new FeatureFlagSet
+func NewFeatureFlagSet(flags ...string) FeatureFlagSet
+```
+
+When parsing the `--features` flag, the entire `[]string` will be passed to `NewFeatureFlagSet`.
+Additional features can be added with the `Enable` function.
+Parsed features will be printed as an `Info` level message on server start up.
+
+No verification of features will be done in order to keep the implementation minimal.
+
+On the client side, `--features` and the `features` key in `config.json` file will be additive, resulting in the union of both.
+
+To disable a feature, the server must be stopped and restarted with a modified `--features` list.
+Similarly, the client process must be stopped and restarted without features.
+
+## Alternatives Considered
+Omitted
+
+## Security Considerations
+Omitted