Blacklist vimwiki filetype

[vimwiki][] has a markdown-like syntax, which doesn't bode well with YCM
(flickering "pattern not found" messages, performance hits with long prose).

[vimwiki]: https://github.com/vimwiki/vimwiki
Update filetype blacklist documentation

Follow existing `Default: '[see next line]'` style to avoid an overly long line
Merge pull request #626 from tlvince/blacklist-vimwiki

Blacklist vimwiki filetype