[DO NOT MERGE] Global LS Command Registry to register/unregister and execute commands

[BoykoAlex]

Fixes #212 Introduces global LS commands registry where commands are registered. Registry is shared between all LS clients and can be accessed to execute commands. AtomEnvironment has CommandRegistry but it's unclear how to utilize it for LS client purposes... Seems like commands in Atom CommandRegistry are mainly for the UI (i.e. click or pick command in the command palette) since they don't take parameters...

Don't think this is the final solution for the problem, but maybe it's a starting point to gather some feedback and turn this into a shape of a PR that can be merged.