New arguments args and build_args to release()

Allows e.g. to call `devtools::release(build_args = "--manual")` for packages that need a pre-built manual.

Fixes #603.

~~Closes #607 (=contains it).~~
Your version is certainly shorter, but I fear that it might take longer for a human to parse that code when trying to figure out what it does. I've reformatted the code to match the other conditional extension of `args` that happens below.