Add extensions for ConTeXt

[ConTeXt] is a macro language build on TeX (just as LaTeX is build on
TeX). It tends to use `.mkii` and `.mkiv` extensions to represent files used in
Mark II (MkII) version and Mark IV (MkIV) version of ConTeXt.

[ConTeXt]: http://wiki.contextgarden.net/
Also add `.mkvi` file extension
Merge pull request #612 from adityam/master

Add extensions for ConTeXt