v0.2.0

Refactor to fix several issues.

• Made sure whitespace changes are not recorded in undo. Refs
  #1
• Made sure trailing whitespace is not removed in Markdown
  (source.gfm). Removing trailing whitespace in other scopes is
  convenient, decided to keep, maybe will add a setting later. Refs
  #2
• Made sure multiple cursors are handled properly. They weren't before.
• Made sure the last line of the file gets special treatment to not add
  the indentation whitespace that cannot be removed.
• Made sure the indentation whitespace is removed before saving the
  file.
• Provided activate/deactivate methods. The package behavior didn't
  stop when the package got disabled via Settings.