Fix for issue #93, in which blank lines are indented when a selection of...

... lines are indented. This prevents that and adds a regression test for it to test_editorcommands.py

Given your comments on issue #93 I thought it best to submit now a pull request for the fix I have. Separate to this I can investigate as to whether it would be fruitful to implement `indent` in terms of `_indent_line`.