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