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.
Except for the very last related Lua patch (which adds "p" and which logic I didn't try to understand fully), I followed the entire code and kept the behavior identical consciously.

So while I did start with copy/paste, it didn't stay that way blindly. I do think that there's value in keeping the js/lua behavior and implementations as close as possible to allow easier concurrent maintenance and less gotchas for script authors due to minor behavior differences.

I can drop deprecated behavior though, if you let me know which aspect of it is deprecated.