Feature request: Add calligraphic symbols

Calligraphic symbols (`\mathcal`) are available in Unicode (https://en.wikipedia.org/wiki/Mathematical_operators_and_symbols_in_Unicode#Mathematical_Alphanumeric_Symbols_block) and are supported by other LaTeX input packages (https://github.com/clarkgrubb/latex-input). It will be really useful to include those here.

Could use `\C<symbol>` instead of `\mathcal{<symbol>}` as shorthand.