Rex table search

Add regular expression matching to Rex::Ui::Text::Table rows.
Table initializers can pass a string assigned to the 'SearchTerm' opts hash key which will be converted to /#{string}/nmi. This regexp is matched during row output in to_s and to_csv methods in the class. As a result, this approach works for printing full tables with headers as well as creating CSV for -R flags where appropriate.

To demonstrate the functionality, the meterpreter ls, ps, and netstat commands have all be wired with this functionality (netstat is not a fun read when the blue team is looking to off your session). 

Example - find a powershell running as system (likely a bad thing if its not yours):
```
meterpreter > ps -S powershell.*system


Process list
============

 PID   Name               Arch  Session  User                          Path
 ---   ----               ----  -------  ----                          ----
 1016  powershell.exe     x86   0        NT AUTHORITY\SYSTEM           C:\Windows\syswow64\WindowsPowerShell\v1.0\powershell.exe
 5872  powershell.exe     x86   0        NT AUTHORITY\SYSTEM           C:\Windows\syswow64\WindowsPowerShell\v1.0\powershell.exe
```
Example - find processes listening on UDP 500:
```
meterpreter > netstat -S udp.*\:500

Connection list
===============

    Proto  Local address    Remote address         State        User  Inode  PID/Program name
    -----  -------------    --------------         -----        ----  -----  ----------------
    udp    0.0.0.0:500      0.0.0.0:*                           0     0      1000/svch
    udp6   :::500           :::*                                0     0      1000/svch
```
Example - identify all the batch files in Windows' temp folder:
```
meterpreter > ls -S bat$ '%windir%\temp'

Listing: %windir%\temp
======================

Mode            Size       Type  Last modified              Name
----            ----       ----  -------------              ----
0000/---------  105055712  fif   1973-07-02 16:27:28 -0400  JEje.bat
0000/---------  105055712  fif   1973-07-02 16:27:28 -0400  NjIAaSM.bat
0000/---------  105055712  fif   1973-07-02 16:27:28 -0400  WyYeENy.bat
0000/---------  105055712  fif   1973-07-02 16:27:28 -0400  fjurgK.bat
0000/---------  105055712  fif   1973-07-02 16:27:28 -0400  kehsG.bat
0000/---------  105055712  fif   1973-07-02 16:27:28 -0400  zQzOl.bat
```