For consistency, update the value of enable_attributes to be the default
one.

Develop/url image substitution

See merge request pdsl/lektor-python-markdown!3

Alot of modifications was required to make use of `url_to()` during
rendering of the markdown. This was required to make sure we have a
context when executing the rendering.

Fix the plugin description to replace the old description imported from
asciidoc.

Fix default extensions.

See merge request pdsl/lektor-python-markdown!2

Develop/gitlab

See merge request pdsl/lektor-python-markdown!1

Make use of setuptools_scm to manage version.

Following recommendation made by nixjdm, make the plugin configurable
using a config file. Load configuration for the engine, extensions to be
enabled and also load their respective configuration.

Add note to the README file about incompatibility with `markdown-*`
events and any plugin build around them.

Replace all the call to asciidoc by a call to python-markdown.