Add associated args --new and --old

[marksabbath]

This PR introduces 2 new associated arguments to the search-replace command in an effort to resolve the bug reported on #132

With this approach, if users need to replace strings starting with more than one dash (i.e --mystring) they will be able to do that by using the new associated arguments: wp search-replace --old='--oldstring' --new='newstring'.

[schlessera]

We need to check whether both $old and $new were provided by the user, in any combination.

As you've probably noticed, we cannot check this based on the value of the arguments, as an empty string is a valid value (at least for $new), so we have to check based on the presence of the arguments/flags instead.

Also, we'll need functional tests to ensure the flags work as expected.

[marksabbath]

We need to check whether both $old and $new were provided by the user, in any combination.

As you've probably noticed, we cannot check this based on the value of the arguments, as an empty string is a valid value (at least for $new), so we have to check based on the presence of the arguments/flags instead.
Great point @schlessera I'll work on that.

Also, we'll need functional tests to ensure the flags work as expected.
Yeah, I rebased with that purpose. I'll work on that on my next round :)

[marksabbath]

Hey @schlessera would you mind taking another look at this PR?

[danielbachhuber]

Is it possible to update your branch so the changes to .github/* don't appear in the diff?

[marksabbath]

You mean rebase it? I did that before pushing, but will do that again.

[danielbachhuber]

I don't love that we're changing the behavior of the positional arguments to fix this very small edge case.

Might it be possible to handle this in wp-cli/wp-cli somehow? Better handling of positional arguments that look like associative arguments?

Alternatively, is there some other workaround that could be employed for this edge case?

[marksabbath]

It's been quite a long time since I did the tests, but I do remember that I couldn't find a way to make that work properly without using this approach.

Maybe a change under wp-cli/wp-cli making it aware of which are the accepted associative args and use it as positional if it does not recognize it?

[danielbachhuber]

Proceeding with wp-cli/wp-cli#5594 for this repository. I've captured this PR to https://gist.github.com/danielbachhuber/6c398217cff94e8a23a94b6b23aca3aa in case this PR is auto-closed or broken in some way.