• parses math environments, as titled says
• test that checks you can search/replace within math environment
• sets parser mode according to math or not

notes:

• for now, special math characters like ^ or _ do not induce special behavior. In the future, these should associate super/subscripts with the correct group, accordingly.
• the mode is not actually used. This should throw errors if an underscore is left unescaped in not-math mode.

fixes #106