Add a C++ preparser for Roxygen blocks

This PR introduces two functions:

1. `preparse_block:` takes an Roxygen block as a single string, and produces preparsed (key/value) output,
2. `preparse_file:` takes a file and preparses all encountered Roxygen blocks.
Whoops! I had moved everything to `test-cpp-parser.R` but forgot to remove
this file.


On Fri, Aug 29, 2014 at 12:42 PM, Hadley Wickham <notifications@github.com>
wrote:

> In tests/testthat/test-parser.R:
>
> > @@ -0,0 +1 @@
> > +context("parser")
>
> Are there supposed to be some tests here? :P
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/klutometis/roxygen/pull/275/files#r16917087>.
>