Add ability to enabled/disable components

This is actually a rename of the `noop_flag` API.

* Add a `component()` type that enumerates the components that may be
  enabled or disabled.

* Add functions to the `yokozuna` module to enabled/disable components
  and check if a component is enabled.

* By default a component is considered enabled.

* Honor the search enabled flag in the protobuff search path.

* Add better code documentation and specs.