escape code when embed in case of html entities

[cderv]

This fixes #227

If source_code: embed, the code part is HTML escaped using htmltools::htmlEscape before being included in pre and code tag.
As Rmarkdown code could be composed of some html code too, this is useful so that pandoc does not try to interpret as html and throw an error.

I did not add tests as I am not sure what the way to do it here (only one test file)

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This change is 

[cpsievert]

Thanks @cderv!