Added some basic :delmarks tests
Implemented the :delmarks command

This implements the :delmarks command.   It's not 100% complete because
VsVim doesn't currently support numbered marks.  Getting that support
online after it's added though is pretty trivial

closes #1192
Merge pull request #1225 from jaredpar/fixes-1192

Fixes 1192
Merge remote-tracking branch 'origin/master' into staging