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.
1. Same style issue here. Need alignment.
2. Also, the `conf->base32_alphabet` field is never properly initialized in `ngx_http_set_misc_create_loc_conf`. And this line actually triggers a valgrind error:
```
==9991== Conditional jump or move depends on uninitialised value(s)
==9991==    at 0x480A66: ngx_http_set_misc_merge_loc_conf (ngx_http_set_misc_module.c:485)
==9991==    by 0x430585: ngx_http_block (ngx_http.c:601)
==9991==    by 0x417010: ngx_conf_parse (ngx_conf_file.c:391)
==9991==    by 0x414BEC: ngx_init_cycle (ngx_cycle.c:264)
==9991==    by 0x4079DF: main (nginx.c:333)
```

Anyway, I'll fix both issues myself during the merge. Just FYI :)