Implement '\' escape sequence for demo halfviz's HalfTone language

Current parser implementation forbid users from using curly braces ```{``` and ```}```. This PR addresses that issue by add `\` escape sequence syntax to HalfTone language. Now user can use whatever he/she wants to name a node.

* Use ```\{whatever\}``` will get ```{whatever}```
* Use ```Backslash please don't escape from me. \\``` will get ```Backslash please don't escape from me. \```