include Type1 fonts in database (addresses issue #113)

https://github.com/lualatex/luaotfload/issues/113

* ``pfa` and ``pfb`` can now be indexed
* new ``luaotfload-tool`` option: ``--formats=<...>``; to include type1 fonts in a database update, invoke it with ``--formats=+pfa,pfb``

Demo files:

* loading by path: https://bitbucket.org/phg/lua-la-tex-tests/src/tip/pln-psfonts-1-abspath.tex
* loading by file name: https://bitbucket.org/phg/lua-la-tex-tests/src/tip/pln-psfonts-2-filename.tex
* loading by font name: https://bitbucket.org/phg/lua-la-tex-tests/src/tip/pln-psfonts-3-fontname.tex

Barely tested yet.