feature: remove trailing whitespace for empty lines while typing only

[amcpherson]

I would like a half way point between modifying all empty lines with white space on save, and always retaining white space on empty lines (see #10). An option to remove whitespace only lines while typing would allow new code to be properly formatted without contributing noise to CVS histories. Specifically, pressing <enter> creates a new line with the correct indentation as the previous, pressing <enter> again creates another new line with , but also removes the whitespace on the previous line making it compliant with the standard of having now white space on empty lines.