Add --ignore-unversioned option to revert

From #bzr:

<Adys> Is there a way to force a revert even if it ends up in an error (file not versioned, ...) ?
<spiv> Adys: what error are you getting? revert should always work I think.
<Adys> adys@azura:~/src/bzr/xflight$ bzr revert *.txt
<Adys> bzr: ERROR: Path(s) are not versioned: CMakeCache.txt
<spiv> Oh, I see.
<Adys> i dont see a --force option
<spiv> revert thinks you're trying to revert a file it doesn't know anything about, but I guess what you actually want is for it to ignore the unversioned file(s) matched by *.txt and revert the rest?

This is a reasonable use case. I'd probably prefer to call the option --ignore-unversioned because people tend to have differing expectations of a "--force" option depending on what they are trying to do.

Or possibly revert should just warn and not error about those files?