standalone fontmake executable

Some of you may have noticed that in the latest fontmake release [v1.2.3](https://github.com/googlei18n/fontmake/releases/tag/v1.2.3), I uploaded a stand-alone fontmake executable which bundles everything in a single file and does not even require Python to be installed.

I used the latest [pyinstaller](http://www.pyinstaller.org/) and Python 3.6.1 (official builds from Python.org) from my local machine, with OS X 10.10 and Windows 7 (64-bit).

The two zip files simply contain `fontmake` (or `fontmake.exe` on Windows) plus the LICENSE file.
One can simply download it, unzip and run it in place, or move it anywhere in the shell's $PATH, like `/usr/local/bin`.

E.g., from the Mac bash one could do this:

```sh
curl -LO https://github.com/googlei18n/fontmake/releases/download/v1.2.3/fontmake-standalone-1.2.3-py36-macosx.zip
unzip fontmake-standalone-1.2.3-py36-macosx.zip
./fontmake --help
```

I thought that this could maybe lower the entry barrier and make fontmake more easily accessible for non Python developers, or those are not familiar with pip and virtual environments, etc.

If there's enough interest, I could set up Travis and Appveyor to automatically build the standalone executable upon tagged commits.

Note this is still experimental as I haven't tested it extensively, so I invite anyone who's interested in trying this out and report any issues.

Please let me know what you think.
Thanks