Add multibase support #14

This one is more of a nice-to-have feature, but it could be very beneficial to add support for multibase encoding.

Multibase is a relatively simple encoding that uses a single prefix character to specify the base encoding of the rest of the string. This makes the base-encoded string self-descriptive, without having to specify separately the encoding that was used. Compatibility with non-multibase can be achieved by manually checking the letter prefix to make sure it matches the expected encoding and removing it to get the "clean" string.

A ConvertFrom-Multibase / ConvertTo-Multibase function could be made to support all the base encodings supported within this PowerShell module, starting with Base64 and Base64Url. From there it should be easy to add other common ones like base16 (hexadecimal).