rmarkdown

utils::str

partially revert 7663afa8 and fix the problem at a more fundamental level in pandoc_convert() (let the ambiguous html mean html4)

fix #1213 by monkey patching the highlight.js dependency at runtime (to correct the version and path of the old highlightjs-1.1)

print the structure of the dependency to help debug what exactly was wrong with it (e.g., the error message in #1213 was totally obscure)

fix #1247 and #1154: suppress the error messages during knitr::purl() when trying to evaluate the chunk option `eval` (which is a symbol)