Add support for port references in expressions

[ccrisan]

Is your feature request related to a problem? Please describe:
The qToggle API defines @port_id arguments as port references that can be passed to some functions as arguments. While espQToggle doesn't implement any of the expression functions that accept port references, it would be nice if expressions containing port references could be correctly parsed.

Describe the solution you'd like:
The expression parser should return the error message invalid-argument-kind when stumbling upon a port reference.