Fixes #209, inline tag content is doesn't include nested tags

[khanlou]

Bug/issue #, if applicable: #209

Summary

em, strong, and del tags should include their content in a rich format, rather than just their text.

Dependencies

None

Testing

Run this program:

let markdown = "*Here's some **markdown** with a [link](https://khanlou.com)*"

let html = HTMLFormatter.format(markdown)

print(html)

And observe that the nested strong and link tags are included.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[QuietMisdreavus]

Looks great, thanks!

[QuietMisdreavus]

@swift-ci Please test

[QuietMisdreavus]

@swift-ci Please test macOS