Use tmpile for optimizers

[gregpriday]

It would be better to use tmpfile for our temporary file here in the OptimizeHtml job. This means each optimizer would need to deal with a file handle instead.

Optimizers that need a filename, would just use stream_get_meta_data like this: https://stackoverflow.com/a/7690726

And then just closing the file handle at the end will automatically remove it from the filesystem. It all just seems safer.