Add support for increment/decrement operator to parser

[pjattke]

Our current parser implementation does not support the increment (++) and decrement (--) operators. As they are very common, supporting them would make it easier for users.

Also, currently our parser does not provide a meaningful error message in case that a increment/decrement operator was found. For example:

for (int i = 0; i < 22; i++) ...
unknown file: Failure

stdout:
> C++ exception with description "Syntax error: Expected '=' on Line 4, position 88" thrown in the test body.