Formatting math (much like black formats Python code)

[tdegeus]

Fixes #52

Todo:

• \label{...}, \text{...}, \mathrm{...} should not be formatted.

The simplest is to change the placeholder formatting to not include - (or +, etc.) such that they are not formatted by this function. Then \label{...}, \text{...}, etc could be converted to placeholders before formatting