`editor.getCurrentState` and `editor.setSingleSelectionRange`.
`getCurrentState`: Don't get selection--get only the range, if exists.
Add missing priod in code comment.
Merge branch 'master' into issue_609

Conflicts:
	docs/writing_plugins/api.rst
Merge pull request #609 from wymeditor/issue_609

`.getCurrentState()` & `.setSingleSelectionRange()`