Use actual value for SINGLE_QUOTES/DOUBLE_QUOTES
Add snippets `'`, `"` and `doc`

`'` and `"` use a new function `triple_quotes_handle_trailing` which
handles trailing/existing quotes.

This handles expansion of`"|"`, where the second quote gets automatically
inserted by e.g. delimitMate.
Merge pull request #470 from blueyed/python-doc-quotes

Python: add triple doc quotes and handle expansion of `"|"`