Allow setting TEXINPUTS; document `setCompileCommand`.

Improved the README documentation, and in particular added documentation of
the `setCompileCommand` method.  Added a use of `setCompileCommand` to the
example in `example.js`.

Allow the user to set the TEXINPUTS environment variable to adjust the
LaTeX search path, using `setCompileCommand`.
Slightly improve error handling.

Really we should have a 'quiet' parameter which makes gammalatex *only*
return the stdout/stderr in the Error object.  But this is a start.