renderer: add support for codespan

[radeksimko]

Closes #11

Test cases reflect https://spec.commonmark.org/0.30/#code-spans

The two commented out test cases will panic until/unless the relevant two kinds are implemented or yuin/goldmark#299 is merged.

[teekennedy]

Thanks for doing this! One step closer to full Markdown support.

Re: the commented out test cases. I have some WIP code that should prevent the panic issue regardless of node kinds.

[coveralls]

Pull Request Test Coverage Report for Build 2287634712

• 19 of 19 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 99.677%

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Totals
Change from base Build 2211251460:	0.02%
Covered Lines:	309
Relevant Lines:	310

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💛 - Coveralls