added gate with incoming classical wire

[thomasgwong]

Added \cgate, which is a gate but with a classical incoming wire instead of a quantum incoming wire. This is useful for creating resets, as shown in the bottom-right corner of the following circuit:

[Travis-S]

@thomasgwong Thanks for the contribution! Could you update the docs to include a snippet highlighting this gate?

[thomasgwong]

Of course! I just added the snippet. Here's a screenshot showing how it compiles: