Adds ability to load post modules in msfcli

This is mainly important for normal load testing. It'd be unusual to
actually want to use this functionality with msfcli since post modules
already need established sessions in order to do something.

https://dev.metasploit.com/redmine/issues/8719

@kgray-r7 noticed this today.

## Verficiation

 - [ ] See the rspec tests pass.
 - [ ] Test manually with : `msfcli post/multi/gather/firefox_creds S' and see the description.