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.
The reason I added (and tested) this, is to allow js scripts to interact with the mpv window in a way which could allow scripts to extend mpv in more meaningful ways, like osc.lua does. 

And while this function was tested (and related ones like `get_osd_resolution` etc), I didn't yet write any script to use this in a meaningful way.