Allow configuring cleaning white space on save

This condition is checked when saving if the existing
whitespace-cleanup is set instead of when the when prog-mode is is
entered.  This way it can be over-ridden by either dir-locals or
file-local variables.
Add prelude-clean-whitespace-on-save to README.md

Also fix incorrect indentation in prelude-editor.el changes.
Good catch bbotsov!
Merge pull request #285 from nathanstitt/clean-whitespace-on-save

Make cleaning white space on save an option