Separate md rmd

I have blog posts that actually involve a large amount of computation, or interaction with other software outside R (see [1](http://robertmflight.blogspot.com/2013/06/tim-hortons-density.html), [2](http://robertmflight.blogspot.com/2012/11/hive-plots-using-r-and-cytoscape.html) [3](http://robertmflight.blogspot.com/2012/10/sflapply-vs-lapply-in-r.html) for examples) that I do not want to do re-generation of the markdown from Rmd every time, and also want to do generation of the markdown outside of site updating.

Therefore, I made two changes. One is putting the knitting into another function, knit.post that is exported so that the user can do knitting just like the blog engine does, and two is having the render.post function check for existence of the markdown, and compare the modification dates of the files to decide if knitting needs to be done or not.