remove reliance on knitr:::parse_params()

[jashapiro]

While it is convenient and helpful to use knitr functions where possible to avoid new parsing errors, the use of unexported functions is not a good idea, as these could be changed in a future release.

knitr:::parse_params() is currently used to find whether a code block contains one of the flags that indicates a block should be replaced, but this could presumably be done by a more direct search for the flag options, without fully parsing the parameter list, and without relying on un exported external package code.