CLI tool for quick testing / debugging

[masonium]

This branch adds a small CLI tool, called mathml2text, which will be useful for developers to more quickly implement and debugging changes. The existing main.rs hard-codes a fixed input string and prints a lot of extra debugging output.

For now, it simply takes a file with MathML input, and optionally a language and output format. Text and braille are standard outputs, and audio can be played when compiled with the tts feature.

Future improvements will likely add better tracing of the MathML to text process, such as displaying what rules are used on what elements or showing the intermediate stages of translation (pre-processing steps, processing of subnodes, etc).