Xdoctest

Added
* added `IGNORE_WANT` directive
* added separator between printout of docsrc and its stdout

Changed
* Print correct doctest line number in the traceback
* Runner `on_error` will now default to return instead of raise

Fixed
* Fixed option parsing bug in __main__ script

Added
* The default runtime state can be customized with the `xdoc-options` command line argument.

Fixed
* Fix crash when trying to read binary files
* Fix issue in `_docstr_line_workaround`

Added
* Starting keeping a changelog, all changes before this point are only
documented via the git history.

Added
* Undocumented changes