Added resolve-arg function in selmer.parser which lets you resolve args passed to the handler of custom tags as per request: https://github.com/yogthos/Selmer/issues/303

[didibus]

I added resolve-arg to the selmer.parser namespace, let me know if you think it be better to put it in another one.

I've also followed stuart sierra's advice of not having a function that maps and having one for a single element where it's trivial
for users to map over it.

I added to the README how to use it.

I have 3 failing tests on my machine, but they are all for date filter, when using Locale zh, I get some chinese characters as part
of the output, and the time is 12:00 instead of 00:00. Anyways, I did not bother debugging them, since they are clearly unrelated to
my changes.

I did not update the changelog, I think you might want to do that when cutting a new release?

Regards

[yogthos]

Fantastic, and I think selmer.parser is as good a place as any. And looks like those tests might be JVM version dependent because the formatter behavior changed at one point. All tests pass on my machine, so I think we're ok there. :)

And just cut a new release 1.12.59 with the update! 🎉