Add ubuntu-images extension

[waveform80]

Split out from the ubuntu-boards-documentation. Tests included as doctest. To run, from a venv with the extensions installed:

python3 -m doctest canonical-sphinx-extensions/ubuntu-images/__init__.py

Being doctest, no output and zero exit code means tests have passed. If you have coverage installed:

coverage run -m doctest canonical-sphinx-extensions/ubuntu-image/__init__.py
coverage report

P.S. Please let me split up the README on the front of the repo :)

[waveform80]

flake8's still complaining about line-lengths, but most of these are either:

• difficult to correct (e.g. doctest expected output which includes intentional whitespace and formatting)
• simple to correct but doing so would obscure the intention (the test case generating the release file)
• not worth correcting (e.g. one or two characters over and splitting the line would make it less readable)

Personally I'd recommend just disabling the E501 test (or weakening it considerably). Actually I'd recommend switching the test to pylint (which tends to pick up more useful stuff and has much more descriptive and flexible disable comments), but that would probably be more painful.