Make silk from a sow's ear by converting a plain R script into something that can be knit()'ed with knitr!

Basically, convert an R script from something like this:

## # Heading
## Description
some.code()

To this

# Heading
Description
``` {r }
some.code()
```

which can then be run through knit.

Note that a version of sowsear is directly available in knitr as spin, though with different defaults.