More backwards compatible API

This commit aims to make it easier for existing code to support multiple selections.

- Add doc.withSelection(), which is doc.eachSelection() except inside another call to eachSelection, where it only calls the function once, and make the API use it.
- Support returning CodeMirror.Pass within eachSelection
[matchtags addon] Support multiple selections
[match-highlighter addon] Support multiple selections
[closetag addon] Support multiple selections
[continuecomment addon] Support multiple selections
[continuelist addon] Support multiple selections