batman: add --no-fallback option

[llakala]

If this is an existing feature, apologies! I couldn't find it. I'd go deep into my specific usecase and why this would benefit me, but I don't think that's necessary, as it's a fairly common-sense option.

Currently, a command like this: batman git pasta would fail to find a manpage for git pasta, and would then fallback to the manpage for git. However, this may not always be helpful. Sometimes, you didn't just want a general manpage, you wanted one for a specific subcommand. Rather than having to check if you're in the right manpage, it'd be nice to just see "No manual entry for pasta", and immediately know that the manpage doesn't exist.