Add omni completion source

[cbarrete]

This is a basic source that works, but I think that it should be filetype-specific, because it is only relevant in a few cases (e.g. when editing LaTeX files when https://github.com/lervag/vimtex is installed). However, there isn't a good way for users to configure filetypes for sources right now, since they don't have access to the source metadata.

It would be easy to add a global option and read it in get_metadata but that's quite ugly in my opinion.

Ideally, I'd like to configure filetypes via files in an ftplugin directory (which only works for Vimscript, Lua support is planned for 0.6, which is still far away) or autocommands. I feel that it shouldn't be the responsibility of a plugin to handle filetype management since (Neo)Vim already has a very good way to do that.
I can open an issue to discuss that if you don't have a plan for it yet.

[hrsh7th]

Thank you very much! I really appreciate contribution!

I'm reviewing.

[cbarrete]

Thanks for the review, I was hoping for a discussion about filetypes before going into how the omnifunc works, but I'll just open a new issue.
I think we can merge it for all filetypes as it is now (unless you have more comments of course).

[hrsh7th]

@cbarrete
Sorry. I was missing that point.

In compe, the filetype metadata can be confifured by g:compe.source.omni = { 'filetypes': ['tex'] }

[hrsh7th]

Looks good. I will merge it.

[cbarrete]

In compe, the filetype metadata can be confifured by g:compe.source.omni = { 'filetypes': ['tex'] }

Ah thanks, I didn't realize that, I'll still open an issue as I don't think that it's an optimal solution, but we can discuss this :)