Enhance/add alphabet param

[dannywillems]

This PR adds the possibility to specify different alphabet in the functions to include, for example, the ripple base58 alphabet: https://developers.ripple.com/base58-encodings.html

I also renamed for more clarity the alphabet module variable in BITCOIN_ALPHABET, and add the ripple alphabet too.

I suggest to update the version to 1.1.0 as it could break if people uses base58.alphabet

[keis]

Thanks for the PR @dannywillems

A few minor nitpicks that it would be great if you could fix but otherwise looks good.

• Please squash the pep8 fixes with the commit that introduce the issue and either complete the docstyle change or leave it be.
• Instead of making a breaking change could you add alphabet as a alias for BITCOIN_ALPHABET?
• Update the examples in the README to show how to use the alphabet flag

[keis]

Thanks!

(Sorry for not getting around to this earlier for some reason github doesn't notify on code updates to PRs ...)