Allow a custom alphabet to be used by the encoder

[samkirton]

[mikedizon]

this is great. is it possible to support a character map?

I'm trying to use kai notation

https://github.com/elixireumorg/base32-alphabets#kai https://github.com/elixireumorg/base32-alphabets/blob/master/lib/base32-alphabets/kai.rb

kai notation maps certain characters like l to 0, and uses letters in place of numbers after 9, etc.