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.
Pardon my ignorance, I guess didn't quite understand what you meant by the configuration phase and moved the code to the wrong place.

Did you mean we should initialize the array in ngx_http_set_misc_create_loc_conf and put it in the conf struct directly instead? This actually makes more sense now that I look at it.