Add ++gui option to suite commands to open underlying context in rez-gui

As discussed, it would be a great feature to be able to open the rez-gui for a context directly from a suite command. For example, "maya ++gui" to open up rez-gui with the associated, maya context.

Start by tracing how suite commands parse options and how the rez-gui entry point receives a context. Verify that a command such as "maya ++gui" opens rez-gui with the associated context, and define completion by confirming the existing suite behavior remains unchanged.

Written by the indexing model from the issue text.