Implement Firebase-style rewrite/redirect/header JSON config

[BigBlueHat]

So far we've avoided doing anything out of config files. However, there's a growing interest in being able to add additional headers, redirect things, and rewrite certain URL's before they hit the file system.

PR #365 has some code for doing such a thing with a (currently) unique/custom JSON format. The discussion there led to wanting (and then finding) a couple rewrite-focused JSON formats. The most capable (so far) is the one from Firebase. See some of that discussion at
#365 (comment)

So. Some questions:

• does using a config file "muddies" this project's purity as a command line "only" server?
• are there other (more) standard rewriter formats we should consider?
• are there non-config file based options for rewriting and redirecting similar to the .headers propose in Add .headers support #360?

Having this system would expand the use cases for this server, so it's a bit of a pivot point project-wise. It also has performance impact that will need consideration/optimization.

Thoughts? 😄

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