Added the 'gdefault' setting
Added the unit tests

Added the unit tests for the 'gdefault' setting and substitute command
Implement the 'gdefault' flag

Implement the 'gdefault' flag for :substitute commands

closes #1236
Merge pull request #1264 from jaredpar/fixes-1236

Fixes 1236
Merge remote-tracking branch 'upstream/master'