Ggrep greps <cword> if no argument is given

When doing a Ggrep search, if user omit to specify a pattern argument,
Ggrep will search the word under cursor.
support :Ggrep \<word\>
Merge branch 'search-word'
Merge remote-tracking branch 'jreybert/ggrep_cword'

Conflicts:
	plugin/fugitive.vim