Rex::Text::Ui::Table.new[find_by_colnames]

Add :[] to ...Ui::Table allowing user to pass multiple colnames.
Returns a new table with only those columns and their rows.

Useful when using Rex to filter output, prep CSV, etc.

Testing:
```
t = Rex::Ui::Text::Table.new('Columns' => ['a','b','c'])
t << ['x','y','z']
t << ['p','q','r']
p t['a','c']

a  c
-  -
p  r
x  z
```