Added js function "readfile".

[Erhannis]

I was working in CAMotics and wanted to be able to draw text, which (for Opentype.js) involved opening a (font) file. I couldn't find an existing way to open an arbitrary file in TPL, so I added readfile to the cbang js standard module to read a whole file into an array of bytes. Let me know if there's already a way of opening files that I missed. Also, it might be worth having a discussion about the security implications of permitting javascript code to read arbitrary files without explicit permission from the user.

[jcoffland]

There's a way to read DXF files but not arbitrary files.

It should probably be restricted to only read files in the same directory as root TPL file or below.