the tags should not be empty, e.g. !{}url! does not work; neither should the caption, e.g. !url()!

#623
cosmetic
use CSS classes separated by spaces instead of _ (#623)
remove the check for output_asis() in every single output hook; do it in wrap.character() instead
partially reverted f0ee3f9 in #635: this only affects old-style figures, but in that case, users are still free to use $\Sexpr{number}$

\ensuremath{} was used only for the sake of safety for sci notations; it also makes sense for negative numbers and infinity symbols, but seems to be an overkill to be applied to all numbers

after re-compiling knitr-examples, there are a bunch of git differences; now they are gone
introduced a bug while fixing #636, and fortunately it was caught by example 091
should be more careful about figure alignment: fig.align must be converted to CSS
block code should not start with a new line
make sure all lines will be pasted into one string
do not use @@ on inline expressions, and @(knitr inline)@ does not really work (#623)
uses css classes knitr plot caption instead of a single knitr_plot_caption
make sure blocks next to a figure are separated by \n\n
add textile to description and reflow it
remove ==<!-- -->==; if Richie does not like this, I can revert this commit later
bump versions of highr and formatR