Add support for custom documentation

[tversteeg]

At my work we use a custom documentation format for Lua and C++. I would like to be able to support this.

My suggestion is to add a custom format similar to snippets.

[kkoomen]

Mhm, very interesting feature which could also improve the plugin itself, because now I'm using an array of patterns per filetype, but maybe I can use something like snippets for the base of the plugin which can be extended by the user itself.

I'll work on it.