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).~~
 `c(NULL, 'a')` yields `c('a')`, so would it also be OK to do this?

```R
args <- c(args, if (!"--manual" %in% args) "--no-manual", "--no-resave-data")
```

(note infix operators take precedence over `!`)