Add support for a custom tag prefix

[hawkw]

This branch adds support for specifiying a tag prefix before the version
number. The prefix is specified as a bash regular expression; the string
matching that regular expression is available in the $prefix variable
that can be interpolated into the changelog path and title string.

Additionally, since I was adding support for interpolating $prefix
into the changelog path anyway, I went ahead and allowed $version and
$tag to be interpolated into the changelog path as well. This might
be useful in cases where the changelog for each version is stored in its
own text file...though I don't think this is a particularly common
pattern.

Closes #1

[taiki-e]

Thanks!

---

It's possible that maybe we'd actually prefer to allow interpreting it
as a pattern, so I'd be happy to change that if so.

I was originally thinking of a use case where tags exist for each crate, such as crossbeam, so I think eventually supporting regular expressions might be preferable.
However, in such a use case, there is a changelog for each crate, so supporting regular expressions in the tag prefix is likely to be insufficient.
So, I don't believe that regular expression support will block this PR.

[hawkw]

I was originally thinking of a use case where tags exist for each crate, such as crossbeam, so I think eventually supporting regular expressions might be preferable.

That was also what I was imagining, actually --- my goal was to be able to use the action for the tokio-rs/console repository, which contains three crates and separate release tags for each crate. I was actually planning to use my own regex, anyway, though, to extract the names of the subdirectories for the changelog path; the prefix can then just be a regular string, because I've already extracted the name of the crate: https://github.com/tokio-rs/console/blob/3da945d13cfa7fc95cb643a896006667f0da9972/.github/workflows/release.yaml#L3-L42

It would be even nicer if this action could handle extracting the crate name based on a regex prefix, but that would be a bit more work, I think...we'd then also need a way to express that a variable (the prefix) should be interpolated into the changelog path, the way we currently do for release titles.

[hawkw]

After thinking about it a bit more, we should probably just make prefix: a regex, make $prefix get set to the part of the tag that matched the regex, and allow interpolating $prefix into the changelog path...

[taiki-e]

After thinking about it a bit more, we should probably just make prefix: a regex, make $prefix get set to the part of the tag that matched the regex, and allow interpolating $prefix into the changelog path...

Sounds good to me.

[hawkw]

Okay, the prefix is now matched as a pattern, and $prefix, which contains the portion of the tag matching that pattern, can be interpolated into the title and changelog path.

[taiki-e]

LGTM

[hawkw]

I've replaced the use of expr with grep as you suggested. Let me know if there's anything else I need to do?

[taiki-e]

Published in 1.3.0.