add Tcl language syntax highlighting

Hi! 
I have created a syntax highlight rules for Tcl (http://www.tcl.tk/). I went according to the instructions in http://highlightjs.readthedocs.org/en/latest/language-contribution.html and it seems everything is fine.

Screenshot:
![highlightjs_tcl](https://cloud.githubusercontent.com/assets/6383928/4099189/e0149636-3050-11e4-976d-e91d93c63892.png)