Start emacs mode.

This only implements basic syntax highlighting.
Expose location of bundled emacs mode via CLI.

This adds a command-line option `--locate-emacs-mode` that prints the
full path of the directory that contains the bundled emacs mode to
stdout.

If pts is installed with `cabal install`, cabal will also install the
emacs mode to some appropriate directory on the target system, and
arrange for `pts --locate-emacs-mode` to return the path of the
installed emacs mode.

If pts is build without installing it, cabal will arrange for `pts
--locate-emacs-mode` to return the path of the emacs mode in the source
tree. I tried this with `cabal run pts` and `cabal repl`.
Explain how to set up emacs mode.
Refactor emacs mode to use smaller functions.
Implement C-c C-l.

To call pts, the emacs mode needs to know the project directory and
the PTS instance. If there's a module statement in the file, the project
directory is guessed. If that fails, the user is asked once per buffer.

I don't know how to guess the PTS instance, so the user is asked once
per buffer.
Document keybindings in readme.
Merge pull request #85 from Toxaris/emacs-mode

Add Emacs mode (fix #81)