Adding set_misc_base32_alphabet directive to allow the use of custom alphabets

Added code, tests and documentation for the new directive.

One issue with the code is that it does not check the alphabet length.
In cases where the alphabet is less than 32 chars code should not work correctly.

I was unsure where to do this validation (and throw NGX_CONF_ERROR perhaps?), so any advice is appreciated.
I'd like the directive name to be simpler: `set_base32_alphabet`. Yes, I'm aware of the existing `set_misc_base32_padding` directive and I'll deprecate the old form and introduce the new form `set_base32_padding`, which sounds better :) I'll do this change myself during the merge :)