Support backticks in <code> spans (#226)

[chrispy-snps]

Fixes #226.

When the contents of a <code> tag contain N consecutive backticks,

• The resulting Markdown will use N+1 backticks as a delimiter.
• An additional leading and trailing space are added inside the span to avoid backtick ambiguity.
  • These spaces are ignored by most Markdown parsers.

For examples, see the unit tests included in this pull request.

[AlexVonB]

I did not know `this` is possible, nice!