Within a single Python session anybadge will use different SVG mask IDs.
This is an attempt to fix the issue where SVG files displayed in the same
HTML will re-use the same mask, and therefore appear with the same dimensions.
Uses a singleton on the class to maintain a sequence of mask ids.
Includes unittests to ensure new ID is used for each badge.
* Add __repr__ function that prints the object representation.
* Add __str__ function that prints the Badge SVG text.
Also:
* Add constructor args to the Badge class docstring.
* Change the way defaults are used in the Badge constuctor. Instead of
setting values, use None and set arguments to the default values inside
the constructor.
* Fix unittests that don't use overwrite=True.
* Use a better estimation technique to guess text widths.
* Add unittests for validating badge widths for various size badges.
* Add return types to function docstrings.
* Update relevant doctests.
