Use os.tmpdir() instead of .tmp

[ericclemmons]

(I can send a PR for this if you'd like.)

Until #25 lands (which I think is great, but complicated), rather than .tmp/, we could use require("os").tmpdir(), which does what it says :)

[ericclemmons]

Or, alternatively, give preference to output.path, if it exists.

[zinserjan]

Hmmm, I think that it's not worth it to invest time on this.

Also, when we move the tmp directory to systems tmp dir, this could lead to problems with webpack's externals config. Node's require algorithm will no longer find node_modules.

[ericclemmons]

Makes sense. Thanks!