Fix to parse _n and _nx, add command line option --translation-domains for filtering translations.

[chrono-meter]

Fixed a bug in parsing _n and _nx where the argument order was handled incorrectly.
Also, if parsing fails, an error is output and no entry is generated. This is probably the correct behavior.

Added command line option --translation-domains.
This is useful when we want to use translations for our own domains in combination with translations for the default domain.

[chrono-meter]

The descriptions of the command line options --domain and ignore-domain should be changed.
The addition of --translation-domains will confuse users.

[erikyo]

Ok, thank you! I'll try to take a look at it as soon as possible. I just ask you to be patient in the meantime.

As for the descriptions of the commands you mention (domain, ignore-domain), do you have any proposals on how?

[chrono-meter]

Sorry for the late reply.
Something like File domain might be good. I was initially confused because I thought --domain was equivalent to --translation-domains.
However, it might be enough to just add the explanation Valid domains are: plugin, theme, block, theme-block, generic to the command line option help. It's from current error description.

[chrono-meter]

I've made some fixes to include your comments.

And I've modified the description of --domain command line option, but only to note the available values.

[erikyo]

@chrono-meter really a nice job! Thank you again for your contribution!

ps. please next time (because i hope there is a next time 😉) make you modification in a new branch, and push multiple PR if required. In this case, since there were two different changes, I would have preferred two separate PRs, one for _n and _nx and the translation domain. It's no big deal, I do the same thing, but from the reviewer's point of view, it would have been clearer to have two separate ones.