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']
t['a','c']

=> a  c
-  -
p  r
x  z
```
whitespace cleanup
Merge for retab
Retab changes for PR #1681
Merge pull request #11 from tabassassin/retab/pr/1681

Retab/pr/1681
fix indentation
Use snake_case for variables
Check for inexistent column names
Land #1681, @sempervictus Rex::Text::Ui::Table [] method