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

 - [x] See the rspec tests pass.
 - [x] Test manually with : `msfcli post/multi/gather/firefox_creds S` and see the description.

## Example:

````
[ruby-1.9.3-p484@metasploit-framework] (bug/8179-msfcli-load-post) 
todb@mazikeen:~/git/rapid7/metasploit-framework
$ ./msfcli post/windows/gather/cachedump S
[*] Initializing modules...


       Name: Windows Gather Credential Cache Dump
     Module: post/windows/gather/cachedump
   Platform: Windows
       Arch: 
       Rank: Normal

Provided by:
   Maurizio Agazzini <inode@mediaservice.net>
   mubix <mubix@hak5.org>

Description:
  This module uses the registry to extract the stored domain hashes 
  that have been cached as a result of a GPO setting. The default 
  setting on Windows is to store the last ten successful logins.

References:
   http://lab.mediaservice.net/code/cachedump.rb



[ruby-1.9.3-p484@metasploit-framework] (bug/8179-msfcli-load-post) 
todb@mazikeen:~/git/rapid7/metasploit-framework

````