Coq 8.4 support

[leon-vv]

It looks like the master and pathogen-bundle branch are currently being updated to support Coq 8.5. However this causes vim to crash for people using Coq 8.4, like me. Since Coq 8.4pl4 is currently the version shipped with Debian stable I think it's worthwhile to have it supported. I saw the tag '8.4' pointing to commit '30a8f16' which did not work on my machine. The merge commit '3bd17ca' however worked great.

If we want to support different Coq version I propose using a branch for each minor version (8.4, 8.5, 8.6) and feature branching. We can then remove the pathogen-bundle branch and let the master branch contain all non-version specific code. What do you think?

Thanks for the project by the way, it's great!