👌 IMPROVE: External links in new tab (#856)

[marjuscako]

Add support to enable external links to open in a new tab. Expose the configuration option myst_links_external_new_tab, which if set will set all URL links to open in a new tab on the browser.

Besides that, we allow the user to set the target and rel attributes when using the extension inline_attrs (#820).

Closes #820 Closes #856

[welcome[bot]]

Thanks for submitting your first pull request! You are awesome! :hugs:
If you haven't done so already, check out EBP's Code of Conduct and our Contributing Guide, as this will greatly help the review process.
Welcome to the EBP community! :tada:

[marjuscako]

Thanks a lot for the pointer @chrisjsewell . I updated my commit with a couple of tests for these features. Please let me know if that will suffice!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.10%. Comparing base (abcc087) to head (06a63f3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #857      +/-   ##
==========================================
+ Coverage   90.09%   90.10%   +0.01%     
==========================================
  Files          24       24              
  Lines        3372     3377       +5     
==========================================
+ Hits         3038     3043       +5     
  Misses        334      334              

Flag	Coverage Δ
pytests	90.10% <100.00%> (+0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[welcome[bot]]

Congrats on your first merged pull request in this project! :tada:
Thank you for contributing, we are very proud of you! :heart: