it is a common misconception to write ```{r, options}, so I'll surrender to it

if there are empty commas in the beginning or the end, strip them off
white spaces have been stripped off, so just check params==''
rename knit_env.Rd to knit_global.Rd
hide knit_env() because it is rarely useful, and export knit_global() instead; fixes #456
factor out known global objects in find_globals(), and add `(` to the list
roxygenize
mention changes of knit_env() and knit_global() in the news