Add label_formatter callback

I added a label_formatter callback to pie.rb.  For example, to substitute the "label" for the "value" in a pie chart, use this:
<pre>
   g = Gruff::Pie.new("350x180")
   g.label_formatter = Proc.new { |data_row| data_row[0] }       # Select label from [label, value, color ]
</pre>
This can be used to format numbers or substitute text.
I apologize for the whitespace changes that confuse things. This is my first github contribution.