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.
Please ensure C variable declarations are at the beginning of the current code block. This `for` loop breaks this rule.

Also, I'm wondering if it's possible to initialize this `basis32` array at the configuration phase so that we don't have to do this over and over again upon every invocation of the decoder in every request.