Allow -o, --only-matching work together with -r, --replace

[beyondcompute]

Hello!

Thank you for your amazing work on much-needed and super-fast fully-featured regex search utility which is ripgrep.

However I’ve been missing a bit a feature similar to ack’s helpful --output which allows printing only certain capture groups. That feature is discussed in #320 and the discussion offers a solution. However it requires matching an entire line, which still might seem a bit less straightforward for some users.

Now, there is probably a reason why -o is made to conflict with -r so please excuse me if this PR is really out-of-place. However after implementing it, upon superficial gaze, things seem to be working fine:

Examples outputs

ripgrep

$ ./target/debug/rg 'of (\w+)' -Nor '$1' ./tests/hay.rs 
this
detective
luck
straw
cigar

ack

$ ack 'of (\w+)' --output '$1' ./tests/hay.rs 
this
detective
luck
straw
cigar

So I hope this has a chance of being considered. Thank you again!

[BurntSushi]

I can't quite remember why we set the flags to conflict. There might be some subtle corner cases. I'd have to go back to that PR and read.

With that said, I'm not in principle against this, and I feel like this combination of flags makes sense.

[ayosec]

I can't quite remember why we set the flags to conflict.

I guess that the conflict was set because the original implementation didn't add the if self.only_matching condition in the if self.replace.is_some() block.

The combination of -r and -o seems pretty natural for me. Actually, I tried to write rg -o 'Enqueued (\S+)' -r '$1' a few minutes ago, and then I saw that it was impossible :-( .

[BurntSushi]

I rebased this on top of master and merged it in f887bc1. Thanks!