Apply middleware before all served assets.

[brettburley]

This enables configured middleware to apply to all served assets, including those served from the public/testem folder.

This is intended to address #1784.

[johanneswuerbach]

Hey, thanks for the contribution. Would you mind to add a test here https://github.com/testem/testem/blob/master/tests/server_tests.js#L116 that ensures we don't regress? Maybe use a static middleware that adds an additional response header to all responses and check that is added on static files?

[brettburley]

Thanks, good call! I went ahead and added test assertions for the middleware to all of the request types where middleware should be applied. Let me know if you have any feedback.

[johanneswuerbach]

Thank you @brettburley , published as testem@3.13.0