Add compilation settings

There are two sets of settings, "file", hooked up to eval.one, and
"project", hooked up to eval.  This required a significant rewriring of
the eval process; notably, saving no longer triggers an eval by itself
(since we don't know which to use).  Settings are stored in
~/.config/litexrc and in .litexrc in each directory.

One of these settings is the PDF file produced, so it's no longer so
obvious which to use for forward syncing.  After compilation, we store
the file name in the editor and use that.  Before any compilations, we
use the filename from either the "file" or the "project" settings, if it
exists; otherwise, we bail out.

Ref #3.
Check the pdf filename when forward syncing, and load if necessary
Remove "directory" from settings

It was untested, and I don't think it would have worked in a
particularly useful way.  Instead, we take the directory of the filename
to be the working directory.

Also, simplify the figuring of the pdfname in get-config-from-settings,
since we should always have a settings value for "outputname" now that
synctex is handled elsewhere.
Add support for non-TeX files

If you try to compile a non-TeX file, it will find the last TeX file you
compiled, and compile that instead.  This is set up for BibTeX files.
Update README with info on settings

Closes #3.
Catch error parsing JSON settings files
Zoom PDF viewer with Alt-mousewheel

Ctrl-mousewheel would be preferred, but it looks like node-webkit
doesn't let those signals through.
Add commands to edit LiTeX settings
Version 0.0.3