Add formula for eSpeak

Adds a formula for the [eSpeak](http://espeak.sourceforge.net) text-to-speech tool.
[There is](https://gist.github.com/stewart/06b3204018d70c3333bd#file-makefile-L134-L152), the script as-is just installs the executable and necessary data to run it, but not the library or header files.

Would it be preferred to use `make install` instead to install everything?