Add support for input from stdin

Open Nasir Hemed requested to merge nasirhemed/dav1d:add-stdin into master

Fixes issue #286

Allow the user to run dav1d by piping input from stdin. To do this, specify -i - and the stdin will be piped

Since there are multiple areas where a file is opened and the fact that there is seeking involved, the workaround is to create a temporary file and put everything from stdin to that temporary file. After that, we continue processing in the same way as a file and remove the temporary file at the end.

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