Merge with NicoSantangelo/SublimeLinter-contrib-flow

Hello! sorry for the delay!

I finally got around [merging](https://github.com/NicoSantangelo/SublimeLinter-contrib-flow/issues/2) the packages. I've splitted the commits in the different features so you can review them separately (and if you don't like any of them, rebase and remove them).

Try it out if you like! if anything looks suspicious let me know, I'm happy to help.

Thanks! 
Actually, it doesn't need to use 'File-only' mode but the config_file line still isn't needed. I'll remove line 48.