Add support for knitting in alternate directories

[jmcphers]

This change addresses one of the largest points of confusion surrounding the notebook release; specifically that executing R Markdown chunks interactively used to evaluate the code in the current working directory, but now evaluates it in the document directory.

After the change, it's possible to choose (both per document and globally) where you'd like the chunks executed. For example:

The chosen directory is used to evaluate chunks both when the document is rendered with R Markdown and when chunks are run interactively. These new preferences are roughly an interactive analog to the new global knitr.use.cwd option. It's worth noting that this parallel implementation does not read knitr.use.cwd, and takes precedence over it.

The new UX is only displayed and used when a suitable version of the rmarkdown package is installed (i.e. one with support for the knit_root_dir parameter).

[jjallaire]

There are so many people that will be thrilled by this change!

I might be in favor of collapsing the menu items into a single "Knit Directory >" menu which cascades to:

Document Directory
Project Directory
Current Working Directory

[jmcphers]

I like that; it's now implemented on the branch.