Allow re-editing malformed JSON

Hello,

My main problem with the gem is that I lose edits if I make typos when editing data bags and try to save.

This modification allows the user to re-open the editor with the malformed JSON in order to fix it.

Implementation note: in order to get hold of the malformed text, I have substituted use of Chef::Knife::UI#edit_data with a new private method: Chef::Knife::SoloDataBagEdit#edit_text. This changes allows #edit_content to get the actual text returned from the editor, which permits re-editing if it's malformed.

Regards,
Joe