Add Javascript scripting support using MuJS

Implements JS with almost identical API to the Lua support, except:
- The global mp, mp.msg and mp.utils are always available
- On error return undefined and the error is at mp.last_error_string
- Timers are JS "standard" global set/clear Timeout/Interval/Immediate
- Supports CommonJS style modules/require
- At mp.utils: read_file (any), and load_file, run_file (for js files)
- Global print and dump functions
- mp.options currently not supported

Requires MuJS (clone, make, make install):
- http://mujs.com/ - git://git.ghostscript.com/mujs.git
- Mirror - https://github.com/ccxvii/mujs

Note: MuJS is a new project and this is probably its first major use outside
of MuPDF. Please keep an eye open for bugs.
This file reading code is pretty messy. Why does it reject unseekable files? It'd be better to read the file in increments. Anyway, the file reading should be in a separate function