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.
Yes. For instance, for `mp.commandv("show_text", "hello");` :
`length` is 3 (index 0 is `this`, and then the call arguments are at indices 1 and 2).

The line just below the quote is `args[i] = js_tostring(J, i + 1);` so it maps indices 1, 2 which are the arguments to `commandv` to `arg` indices 0, 1.