Loosen regex to compensate for iskeyword

Closes #332.
Add foldenable to fugitive_diff_restore
Revert "Add foldenable to fugitive_diff_restore"

This reverts commit cbb7c3e89026472dfbc5440d78ce85975423bedd.
Encapsulate restore options for diff

This should make it easier to experiment with solutions for issues such
as #257.
Press - on first line of :Gstatus to edit .git

This is a bit silly, but provides a quick way to start navigating the
repository.  Press it a second time and you're in the work tree root.
Change heading style
Extract contribution guidelines
Update :Gblame explanation in README
Fix :Gstatus dp with external diff enabled

Closes #341.
Changed a mapping.

With me using a homemade Colemak layout, this mapping resulted in
frequent movement lag.