Downgrade to GitHub Java API 2.1.2

This is only temporary until 2.1.4 syncs to Maven Central
and then that version will be used.
Markdown files render in commit view

Now when you click on the header for a markdown file in the commit
list view it acts the same way as if you were viewing the file in
the source tree view (i.e. the file can either be rendered or shown in
its raw form)

Closes #343
DRY up markdown extension checking

Create new utils package class to check if file name
has a markdown extension.