Add an option to disable cache control headers

While the options `st` offers for cache control are good enough for most use-cases, there are some that it is not able to handle -- for example, setting any sort of custom header here other than `public` or `public; maxAge=X`. Since `st` ends the request after serving a file, the user has no way to customize cache headers any further after `st` runs, and if the headers are set before, `st` clobbers them before serving the file.

This PR makes it such that if you set `cache.content.maxAge` to `false`, it does not set cache control headers, allowing the user to set them beforehand without the headers being overwritten by `st`.

I'm not totally familiar with your style of writing js or testing, but I did my best to conform to both, let me know if there are any changes I can make to improve this, or if there's another implementation you'd prefer and happy to modify!