Show selected auxiliary action

**Feature description:**

```show options``` now displays the selected action when an auxiliary module is being used. Previously, it would show only the selected target if an exploit module was being used.

**Verification steps:**

- [ ] ```use``` an auxiliary module with actions
- [ ] ```show options``` and see that the selected action is displayed
- [ ] ```use``` an auxiliary module without actions
- [ ] ```show options``` and see that no selected action is displayed
- [ ] ```use``` an exploit module
- [ ] ```show options``` and see that the selected target is still displayed

**Sample run:**

```
msf auxiliary(chromecast_youtube) > show options 

Module options (auxiliary/admin/chromecast/chromecast_youtube):

   Name     Current Setting  Required  Description
   ----     ---------------  --------  -----------
   Proxies                   no        Use a proxy chain
   RHOST                     yes       The target address
   RPORT    8008             yes       The target port
   VHOST                     no        HTTP server virtual host
   VID      kxopViU98Xo      yes       Video ID


Auxiliary action:

   Name  Description
   ----  -----------
   Play  Play video


msf auxiliary(chromecast_youtube) > 
```
Literally a careful copypasta and edit of https://github.com/wvu-r7/metasploit-framework/blob/feature/show_options/lib/msf/ui/console/command_dispatcher/core.rb#L3208.