rotunicode should also provide transformations beyond the BMP

Many Unicode errors in applications stem from software assuming, that Unicode ends with U+FFFF (see, e.g., MySQLs misnamed `utf8` charset).

It would be great for testing, if rotunicode could provide an option to switch to those astral Unicode characters.

A set, that (almost) fits like a glove, is found in the "Mathematical Alphanumeric Symbols" block:

https://codepoints.net/U+1D400..U+1D433,U+1D7CE..U+1D7D7

I'd love to provide a pull request for it, but I am uncertain as of how to add this to the existing code: extra parameter to `rotunicode.RotUnicode.encode()`? New encoder `rotunicode.RotUnicodeAstral()`?