Fix link resolution for image to use PRIMARY_ALT

See merge request pdsl/lektor-python-markdown!8

Images/Attachments are only generated into the primary alt folder and
not into the secondary alt. So image url need to get resolved using the
primary alt.

Rebuild the html when context changes

See merge request pdsl/lektor-python-markdown!7

Fix link resolution to have similar behavior to default Lektor

See merge request pdsl/lektor-python-markdown!6

Lektor mistune markdown is not doing any link resolution. Let have
similar behavior.

Fix link resolution by making use of context

See merge request pdsl/lektor-python-markdown!5

Develop/fix link

See merge request pdsl/lektor-python-markdown!4

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.