Allow build_rmds to use customs 'dir_blog' and 'dir_src' folders.

[ErickChacon]

This allows to locate your Rmd files on a folder dir_src (default: ".") and your blog in another folder dir_blog (default: "content"). This add flexibility for different workflows. In particular, I am using blogdown to keep documentation and reproducibility of a project. An example of the structure is as follows:

.
├── data
├── docs                # web folder (includes `content`, `public`, 'static', ...)
├── Makefile            # makefile that uses `builds_rms` for each file
├── scripts             # Rmd scripts folder
└── src

Using blogdown:::build_rmds with parameters dir_src = "scripts" and dir_blog = "docs", will generate and move the files as follow:

scripts/clean/rainfall.Rmd    --> docs/clean/rainfall.Rmd
scripts/clean/rainfall.html   --> docs/content/clean/rainfall.html
scripts/clean/rainfall_files  --> docs/static/clean/rainfall_files
scripts/clean/rainfall_cache  --> docs/blogdown/clean/rainfall_cache

This is very useful because now I can use blogdown:::build_rmds inside a Makefile that defines the dependencies between my Rmd files and updates only what needs to be updated.