fixes #791: do not pass arbitrary args to pdf(), because some may not work

e.g. family='Arial' works in cairo_pdf() but not in pdf(), although both devices have the family argument

the bug was introduced by e430c60d7da595b62455a946f8b27d1dc0bd395c
add the chunk option fig.showtext=TRUE/FALSE; closes #799
collapse output blocks if results = 'hold'; closes #798
add \\b to regular expressions in which_sweave()

e.g. <<lprint=TRUE>>= will no longer be a false positive
save a copy of current chunk options before evaluating them; closes #795
cosmetic ## -> #
closes #704: issue a warning if a plot file is to be overwritten
bump version