Adds pytest_markdown_docs_markdown_it for custom markdownit parser

[thomasjpfan]

Issue: Fixes #31

This PR adds a pytest_markdown_docs_markdown_it pytest hook to allow developers to customize the markdown_it parser. This means they can add plugins such as admon_plugin to the parser.

An alternative is to add something like https://github.com/modal-labs/pytest-markdown-docs/blob/main/src/pytest_markdown_docs/_runners.py, but using a pytest hook feels a little more natural with how pytest does things. For example, pytest-xdist has uses a pytest_xdist_make_scheduler to configure a custom scheduler:

https://github.com/pytest-dev/pytest-xdist/blob/f83299cd646965ac0560adddcf7204adde9f486e/src/xdist/newhooks.py#L96-L100