This provides a framework for completer-writers to create
completer-specific commands. I have in mind to use this for the clang
completer to force reloading of a flags module via `:YcmCompleter reload`.
Now there's a nice user-configurable setting for when YCM should trigger
semantic completion. This is very useful for the new omni_completer that uses
data coming from Vim's omnicomplete system.
This was intended to show the full clang output for a given diagnostic,
including notes. But it appears that libclang does not provide this
functionality...
