fix the problems in R-devel `no visible global function definition for ???`

because R CMD check is even more strict about packages attached; basically no packages should be assumed to be attached
use rmarkdown::html_vignette for R Markdown v2 vignettes
further tweaking rmarkdown::html_vignette style:

1. body font-family from Bootstrap (and Helvetica looks better in my eyes);
2. code font-family does not work in RStudio if any font names are specified, so just use monospace;
3. p code is red with an outer box, like Bootstrap;
4. figure captions are italic;
5. larger bottom margin for p and table (10px);
6. use JavaScript to add Figure X to figure captions;
more code chunks in the knitr-intro vignette
do not assign a caption of the form `plot of chunk label` when the output format is pandoc
add an experimental html_vignette(), further tweaking some CSS from the template in rmarkdown, and add figure numbers `Figure X` to figure captions automatically (related to #805)
use knitr:::html_vignette (to whomever wants to use this, you have been warned about :::!)
assets/header.html is no longer useful since we have html_vignette() now
fixes #833: create short captions by cutting off the captions at '.', ':', ';' or '\'

so that when the caption contains, e.g. \foo{a.b.c}, the short caption will not blow up as \foo{a
replace.assign -> formatR.arrow (a change in formatR 1.0)
the syntax highlighter has been moved to highr
more on #833: \foo{abc} is okay; only \foo{a.:,b} is bad

added $ as a new possible separator for short captions, because of $1.23$
bump version