Xdoctest

Fixed
* Fixed python2 unicode error in collection phase

Added
* Added nocolor command line arg
* Added parserkw arg
* Python 3.7 support

Changed
* Better error messages when you forget a raw string on a google block with newlines in the docstr.
* Tests for malformed google docstr case.

Fixed
* Fixed install issues (/introduced hack FIXME later)
* Fixed issue with raw string lineno parsing

Added
* Added config option for lineno offsets. (corresponding arguments added to
native and pytest runners)
* Partial support for Python 3.7


Changed
* Generally Improved doctest error reporting
* Includes better coloring for quick visual inspection
* Line numbers are now reported in a more intuitive way
* We finally removed the exec `DoctestExample.run` from the traceback!

* (we report line numbers of errors in a more intuitive way).

Fixed
* Fixed GH20 where `doclineno_end` was incorrectly parsed
* Fixed issue where google style block lineno was incorrect

Fixed
* Fixed bug with reporting elapsed time in native runner

Added
* Added auto parsing style. This first tries to use Google, but falls back on
freeform if no google-style doctests are found.
* Errors are no longer printed twice if only a single test is being run.
* Added command "dump" to native runner that reformats enabled doctests so they
can be run as unit tests.
* Implemented delayed want matching. This enables doctests to use multiple
print statements in a row and use a single want statement to check all of
their output.

Changed
* All parsers now default to the new "auto" style.
* Colorized doctest now highlights "want" lines in a distinct (green) color