[PATCH] formatter: make grep command use formatter

[timeless]

>>Hm. What do I do when I have a list, like bookmarks? I could implement something like ``fm.writelist("bookmarks", bookmarks)`` I guess, but I'm not sure
>>
>>whether it's the right approach/desired.
>
> I just noticed the ``deftext`` parameter. I presume it's what handles lists.

Things so far seem to flatten into a string before sending. deftext at
the end of the day is just x % z.

I hit this while trying to format debuginstall. I think for now, just
flatten, and worry about improving list handling later.