Don't Color `--output='$f'`

[DabeDotCom]

The following seems inconsistent:

ack ^root: --output='Matched line $. of file $f' /etc/passwd /dev/null
/etc/passwd       <-- green
12:Matched line 12 of file /etc/passwd
^^              ^^         ^^^^^^^^^^^
yellow          white      green

This PR makes it so only the "metadata" is colored, not the actual --output

[NOTE: This came up while I was working on my forthcoming PR, which will add a --no-lineno option...]