switch CLI to using click, handles printing colors

docs for click are good, would recommend skimming over them
https://click.palletsprojects.com/en/7.x/#documentation

everything is backwards-compatible with the previous
interface, the only minor changes were to the doctor cmd
which can now accept more than one item to run,
and the --skip-config-check to skip the config_ok
check if the user specifies to

added a test using click.testing.CliRunner (tests
the CLI in an isolated environment), though
additional tests which aren't testing the CLI
itself (parsing arguments or decorator behaviour)
can just call the functions themselves, as they
no longer accept a argparser.Namespace and instead
accept the direct arguments

setup.py

@@ -8,6 +8,7 @@ INSTALL_REQUIRES = [
     'appdirs',        # very common, and makes it portable
     'more-itertools', # it's just too useful and very common anyway
     'decorator'     , # less pain in writing correct decorators. very mature and stable, so worth keeping in core
+    'click'         , # for the CLI, printing colors, decorator-based - may allow extensions to CLI
 ]