latex2unicode: Distinguish between regular and math-mode.

This finally allows l2u to render a string which contains math
expressions.  Introduce new step in expand() and new functions
process_math() and massage_input().

Also introduce option-parsing.
t/*: Add --math option to latex2unicode call in tests.
latex2unicode: Introduce multi-line mode!

Another badly anticipated piece is falling in its place.  l2u can now
render a line and break it afterwards below textwidth (new commandline
option).

New functions split_box(), find_splits() and clone_box().
Delete densify().  Its time just wasn't right.
t/multiline.t: Add first, crude multiline test.
t/simple.t: Add another regression test.

For some reason, the macro \rightarrow didn't work during the last few
days while all other tests produced no errors.
L2U/Handlers.pm: Remove \comma handler.

It's a simple constant after all.