Rough sketch of a TLA+ lexer

[thomaslee]

I'm new to writing Rouge lexers and no expert when it comes to the TLA+ language but this does a decent enough job of syntax highlighting basic TLA+ files on my personal Jekyll-based site so thought I'd open it up in case others find it useful.

No pluscal support yet but something I hope to get around to at some point.

[thomaslee]

Added sample and demo files to get tests passing, not sure I fully understand the intent/difference between the demo and sample files but I'll eyeball that a little later.