Add relative path support for Audio and Video Urls to MediaLinks Extension

[RickStrahl]

Allow detect and embed relative URLs for image tag usage of audio and video links. Relates to #715.

Updated linking looks like this:

Note relative links now work picking up media files out of the current folder as well as online links.

[coveralls]

Pull Request Test Coverage Report for Build 5546928794

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 17 of 18 (94.44%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 93.342%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Markdig/Extensions/MediaLinks/MediaLinkExtension.cs	13	14	92.86%

Files with Coverage Reduction	New Missed Lines	%
src/Markdig/Extensions/MediaLinks/MediaLinkExtension.cs	1	96.4%

Totals
Change from base Build 4772706458:	0.02%
Covered Lines:	25987
Relevant Lines:	27242

---

💛 - Coveralls

[RickStrahl]

LOL - sure why not?