use @export instead of @S3method for the next version of roxygen2
closes #733: make the hook name accessible inside the hook function

all arguments of a hook function are optional now

this should close rstudio/rmarkdown#17 as well, because when hook is a character string, I will get() it
pass the encoding argument from knit2html() to markdownToHTML(), which requires markdown 0.6.5

rstudio/markdown#50 @kohske
more Rd changes from roxygen2 4.0
add fig.retina to opts_chunk and default to 1
add collapse = FALSE to opts_chunk
equivalently, use local({}) instead of (function(){})()
add a new chunk option `strip.white=TRUE` to trim white lines in the source chunks, per suggestion of JJ
remove source code blocks that are completely white spaces

this can happen if there are line breaks at the end of a chunk, and the second to last expression is plot()

this removes some empty code blocks in the example 021
roxygenize
bump version
finally closes #484: chunks can use a custom render function
add the knit_print() generic function
instead of evaluate:::render, we use knit_print as the default render function now
roxygenize
bump version
hooks-md.R: Only collapse matched back-ticks.

- Ensure the fence is alone on the line and lengths are equal before
  collapsing.

Signed-off-by: Thell Fowler <thell@tbfowler.name>