Add braille -> MathML functionality

There have been a few requests to support braille input. It's a new direction to take MathCAT but one which has been on my radar for a few years.

Handling braille input would most likely would be done via a parser. [Susan Jolly suggests using a ANTLR 4 parser-generator](https://tug.org/TUGboat/tb40-1/tb124jolly-braille.pdf)... at least for back translation of Nemeth. Rust has ANTLR support, although it depends on a nightly build (seems to depend on `CoerceUnsized` which seems to be moving towards stabilization) and there. There are many other parser generators written in Rust ANTLR doesn't work out.

I suspect implementing support for braille input of other braille codes would work well with this kind of parser approach also. If someone knows otherwise, please comment below.

I have opened this issue as a place to gather requirements, potential solutions, and feedback from others as to what braille languages for which they would like to see support.

As an example of requirements: what should the output be if the braille doesn't make sense? What should it be if it is partial input? For example, there is a start fraction indicator but nothing is after it? What if there are both start and end fraction indicators, but no fraction line indicator?

Please add your comments!