Provide extension points for extending by other projects

[Majkl578]

Neon support should not assume Nette usage, so it should only provide basic features and provide some extension points, so intellij-nette can extend the features (like #18).

[hrach]

👍

[juzna]

Let's try to be more specific: what kind of extensions points + what interface should each one have?

First would probably be completion provider, but I can't think of its interface yet. Perhaps in future.