Merge pull request #4 from jwchong93/master

I merged with the latest
Change and add some functions for ContextManager
so that it sounds more clear and meaningful
Merge pull request #5 from jwchong93/master

Fixed error that keyword behave as identifier in expression context.
Implement the contextsStack for while operator
Implemented context changing in the nud of if
Merge https://github.com/bingxuan3133/CInterpreter