New argument create_if_missing to load_all

[krlmlr]

If load_all() doesn't find a DESCRIPTION file in the package directory, it is created. As of #803, parent directories will not be scanned for a DESCRIPTION file. #816 explicitly documents and tests this behavior.

Now the behavior of load_all() is difficult to understand and communicate: If a directory happens to contain R, src or data, a DESCRIPTION is created -- if not (and no DESCRIPTION exists), an error is thrown:

> pkg <- tempfile(); dir.create(pkg); devtools::load_all(pkg)
Error: '/tmp/RtmpqMuf1p/file498b5f0c4c48' does not look like a package: no R/, src/ or data directories
No traceback available 
> pkg <- tempfile(); dir.create(pkg); dir.create(file.path(pkg, "R")); devtools::load_all(pkg)
No DESCRIPTION found. Creating with values:

I suggest a new argument create_if_missing that governs this behavior. If TRUE, always create (perhaps via devtools::setup()). If FALSE, never create (but use the usual pkg <- as.package(pkg) idiom). If NULL (the default), the user will be asked if in interactive mode (otherwise same as FALSE).

@hadley: Is this sufficient for the use case you described in your comment?

[hadley]

That sounds good to me (although I'd use NA as the default not NULL, and maybe it only needs to be called create?)

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