Add rst handling

[blink1073]

We switched from md to rst in JupyterLab and we were getting some link rot.

Also allows for relative links to supported file extensions.

[blink1073]

Examples of what this catches:

=================================== FAILURES ===================================
 /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/adding_content.rst: https://github.com/jupyterlab/jupyterlab/blob/master/packages/terminal/typings/xterm/xterm.d.ts
https://github.com/jupyterlab/jupyterlab/blob/master/packages/terminal/typings/xterm/xterm.d.ts: HTTP Error 404: Not Found
 /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/adding_content.rst: documents.html
documents.html: No such file: /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/documents.html
 /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/documents.rst: http://jupyterlab.github.io/jupyterlab/interfaces/_docregistry_src_registry_.documentregistry.ireadywidget.html
http://jupyterlab.github.io/jupyterlab/interfaces/_docregistry_src_registry_.documentregistry.ireadywidget.html: HTTP Error 404: Not Found
 /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/extension_dev.rst: ./notebook.html#how-to-extend-the-notebook-plugin
./notebook.html#how-to-extend-the-notebook-plugin: No such file: /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/notebook.html
 /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/notebook.rst: http://jupyterlab.github.io/jupyterlab/modules/_notebook_src_actions_.notebookactions.html
http://jupyterlab.github.io/jupyterlab/modules/_notebook_src_actions_.notebookactions.html: HTTP Error 404: Not Found
 /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/xkcd_extension_tutorial.rst: README.html#getting-help
README.html#getting-help: No such file: /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/README.html
 /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/xkcd_extension_tutorial.rst: ./extension_dev.html
./extension_dev.html: No such file: /Users/ssilvester/workspace/jupyter/lab/docs/source/developer/extension_dev.html
============ 7 failed, 192 passed, 422 deselected in 70.31 seconds =============