Add customizable variable for temp file location.

Also flipped the default to `'inplace`.

I flipped the default because my experience is that inplace generally works better.

I.e. on Mac OS X `temporary-file-directory` is located beneath /var and /var is a sym link to /private/var. Flymake can have problems with sym links.

Some code sniffer sniffs also fails when running on a file in `'tempdir` because the will look for other files in the same directory.