Custom list of prefixes that should be trimmed

[odashi]

Ref: #86

Currently we hard-coded function prefixes to be trimmed as follows:

https://github.com/google/latexify_py/blob/86e612963ee4c8d92c6812f99dd75e2309bc9735/src/latexify/constants.py#L5

It would be good if the users can set it manually.

[odashi]

This should be an additional features of #81.