CLI improvements

CLI arg/param parsing should support the following:
- reusing payload param definition/processing from within exploits
- relatively easy addition of gadgets/payloads/exploits and interoperability with existing (compatible) gadgets/payloads/exploits
- have a reasonable chance of supporting unforeseen exploits/chains/bypasses/etc with minimal changes
- wider variety of gadgets (non-RCE types)
- arbitrary parameters for exploits/gadgets (exploit target host/port/path, remote load gadgets, file paths/contents, etc)
- nesting of payload objects (for wrapping with bypasses, etc) and binding into nested structures
- annotating payload objects with serialization format and peer/child payload object compatibility
- automatically generate help text that shows allowable values for fixed-option parameters