a quick implementation of collecting metadata as a side effect of printing
a new vignette engine for the rmarkdown package
add a vignette for knit_print(); closes #714, and closes #665

note this vignette uses the rmarkdown vignette engine, beautiful! yeah!!
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>
Merge pull request #743 from Thell/tbf-collapse-fix

hooks-md.R: Only collapse matched back-ticks.
add run_document command; collect rmd_warning metadata
Merge pull request #749 from jmcphers/master

Support for emitting multiple knit_meta objects and selective retrieval
jus tested rmarkdown, and this does not work because of the leading white spaces

Pandoc may treat these spaces as the <pre></pre> syntax (#735)
fixes #735: do not use a PNG snapshot

well, this does not really fix the problem; the root problem is from Pandoc
bump evaluate version
use relative font size for the docs column in the docco classic style
the custom rendering function should accept an `options` argument, which is a list of current chunk options

knit_print() also has to have an `options` argument
add chunk options out.width.px/out.height.px (figure size in pixels instead of inches)
an ugly "fix" due to Pandoc's inappropriate treatment of escaped double quotes
roxygenize
bump version
a potential bug: because list[[i]] = NULL will remove i from list

a safer way is list[i] = list(NULL); why bother? because otherwise out.width/out.height will be removed if NULL, and I added out.width.px/out.height.px in options, then due to R's evil partial matching, when out.width is removed, options$out.width will fetch options$out.width.px; this is the reason of the previous failure on Travis CI
no need to carry over the name attribute
cosmetic