feat: skip over unknown directives

[3-w-c]

Fixes #14, #15

This is my first contribution to anything RST, so it might not be the best approach 😅

Test file:

######################
 Prior research
######################

According to :cite:t:`nutt2009`

Before this PR:

docutils.utils.SystemMessage: :16: (ERROR/3) Unknown interpreted text role "cite:t".

After this PR:

• :cite:p: kept as-is 🥳

Future enhancement could be to output warnings, maybe?

[jdillard]

I tested out this branch and it does a great job of skipping roles, but it seems to remove unknown directives. Example:

-Current contributors
---------------------
-
-Thanks to all who have contributed!
-The people that have improved the code:
-
-.. contributors:: jdillard/sphinx-sitemap
-    :avatars:
-    :order: ASC
+**********************
+ Current contributors
+**********************
 
+Thanks to all who have contributed! The people that have improved the
+code:

So a great step in the right direction, but currently destructive until the directive issue is fixed.

The scope of the PR could also be changed to just skipping over unknown roles if lines 268-269 are commented out, and support for skipping directives could come later.

[FlorianSchwahofer]

Hi @dzhu ,

do you have any ideas how one could enable a handling of custom directives to support them in the formatter? I'm not really familiar with the docutils parser...

Thanks and best regards!