Disable modules that rely on Vim version >= 7.4

Instead of crashing and burning when run on vim version < 704, disable those modules that rely on 704.

* Only disable modules that were explicitly requested in vimrc (and show a message)
* Only add those modules to the defaults if vim version is >= 704

Sorry for noisy commit history, was testing across two machines and pushing/pulling was the fastest way to share.

Motivation for this fix was that it got annoying when pantondoc just error-ed out on without explicitly telling me why. And disabling the whole thing would have removed some of the functionality that does work despite the version requirements (e.g. folding).

Lastly I must admit that I have not been using pandontoc for too long, so if there are cross-module dependencies that would render this solution useless