let me maintain the source hook for Jekyll, and you only modify the output hooks

there was a change to the source hook in knitr v1.4.1: the source code (denoted by the argument `x`) can be a character vector that is not concatenated into a single string; the current version of `render_jekyll()` in knitr 1.4.1 does this correctly, but since your `hook.r` is not synced to mine, the current source code rendering becomes problematic

now I'm just using `render_jekyll()` to keep everything update with knitr, and you only need to maintain the output hooks `hook.o` and `hook.t`; hopefully this fixes the problem, and you no longer need to wrap R code into `{...}`
Merge pull request #36 from yihui/patch-1

let me maintain the source hook for Jekyll, and you only modify the output hooks