Enables a rudimentary interactive powershell prompt

Hopefully @dark0perator @mattifestation @obscuresec can advise how to improve this.

Requires Meterpreter update: https://github.com/rapid7/meterpreter/pull/13

Currently there is a slight bug in that each command has to be entered like so:

[Return]command[return]

Need to work out why the streamreader is ignoring the first return etc.

```
meterpreter > powershell
Process 4800 created.
Channel 7 created.
[*] File written to C:\Users\pentest\AppData\Local\Temp/kgoVNpkU.ps1
Hit Enter, Write Command then Enter again...
PS> 
Get-Service

Status   Name               DisplayName                           
------   ----               -----------                           
Running  AdobeARMservice    Adobe Acrobat Update Service          
Stopped  AdobeFlashPlaye... Adobe Flash Player Update Service     
Running  AeLookupSvc        Application Experience                
Stopped  ALG                Application Layer Gateway Service     
Running  AMPPALR3           IntelÂ® CentrinoÂ® Wireless Bluetooth...
Running  AppHostSvc         Application Host Helper Service       
Stopped  AppIDSvc           Application Identity                  
Running  Appinfo            Application Information               
Running  AppMgmt            Application Management                
Stopped  aspnet_state       ASP.NET State Service                 
Running  AudioEndpointBu... Windows Audio Endpoint Builder        
Running  AudioSrv           Windows Audio                         
Stopped  AxInstSV           ActiveX Installer (AxInstSV)          
Stopped  BDESVC             BitLocker Drive Encryption Service    
Running  BFE                Base Filtering Engine              
```