Add concourse plugin

[mmb]

https://github.com/luan/vim-concourse might be a good addition as a default plugin.

[luan]

@mmb it actually used to be included. But that plugin's regexes for syntax highlighting are pretty slow unfortunately on larger files so I ended up removing it. I've been meaning to re-write them but haven't really found the time to do so. I'm going to leave this open as a reminder though, thanks for pointing it out.