Add --color Grep option to the command dynamically

Add --color Grep option to the command dynamically

* lisp/progmodes/grep.el (grep-template, grep-find-template):
Update the description for <C>.  (Bug#20728)
(grep-compute-defaults): Don't add the --color option to
grep-options.  Only add it to grep-command.
(grep-expand-keywords): Expand the env value opts into <C>.
(grep-expand-template): Replace cf in the env with the opts list,
that can include -i and --color.

* lisp/progmodes/xref.el (xref-collect-matches): Do not remove
"--color=always" from the template, because we don't have to.