Fix rendering of string-valued defaults

[hoodmane]

Summary

Before this patch, string defaults are formatted without quotes around them. However, if the default is a tuple or list of strings, quotes will be included:

default="a" ==> :default: ``a``
default=("a",) ==> :default: ``('a')``

This is a slightly annoying inconsistency, and leads to potential for confusion between default="0" and default=0. Most bothersome is that in the case of an empty string we get:

:default: ````

Which makes docutils angry:

CRITICAL: Unexpected section title or transition: ````

This fixes the trouble by formatting the repr of the default.

On top of #136.

Tasks

• [x] Added unit tests
• [x] Added documentation for new features (where applicable)
• [x] Added release notes (using reno)
• [x] Ran test suite and style checks and built documentation (tox)

[hoodmane]

There's still a CRITICAL: Unexpected section title or transition if a multiple option has an empty list default.

[stephenfin]

You can rebase this now that #136 is merged too.

[hoodmane]

Thanks for maintaining sphinx-click @stephenfin!