Add top of diagram empty string marks as an option

[tomcontileslie]

In output.py, the text option indicates muted strings with an x:

   x           
   ===========
   | | | | O | 
   | | O | | | 
   | O | | | | 
   | | | | | | 
   | | | | | | 

but a number of guitar diagrams also put symbols for the empty strings, e.g.

   x     o   o 
   ===========
   | | | | O | 
   | | O | | | 
   | O | | | | 
   | | | | | | 
   | | | | | | 

This should be an available option in the print functions.