specify and implement behavior for combining --only-matching with --replace

[BurntSushi]

@kpp writes:

How about:

$ rg '(?P<first>[A-Z][a-z]+)\s+(?P<last>[A-Z][a-z]+)'  tests/test.txt -o
7:Baker Street

$ rg '(?P<first>[A-Z][a-z]+)\s+(?P<last>[A-Z][a-z]+)'  tests/test.txt -o --replace '$last, $first'
7:Street, Baker

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@mernen writes:

Guess someone beat me to implementing -o! (Which is fair, I’ve been postponing this since January)

So, as I mentioned in #308, I think the best combination for --only-matching and --replace would be an approximation of Ack’s --output: perform a replace on the matched part, and output only the result of the replacement, not anything else on the line. If there are multiple matches on a single line, output multiple lines. Same as on @kpp’s last comment, if I understood it correctly.

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See also #422

[BurntSushi]

Note that this still needs a specification. Ideally, the specification is in the form of user facing documentation.

[BurntSushi]

This was done in #593.