sketching out the core API

[shiftkey]

I spent a bunch of time with mleech/scotch yesterday and learned a bunch, but wanted to start from scratch with consuming the vcr on-disk format rather than remembering how to F# all over again.

Goals to prove out in tests:

• get this running on appveyor
• read a cached JSON representation out to disk when it exists
• for a missing request, store the received response to disk as JSON
• for an updated test, append the new request to the array and store it to disk

After that, I'll start breaking this up into a proper API.

• write a readme
• write some example code

Other interesting formats that might be useful here are nock (example fixtures) but this is a slightly different variant on JSON storage, so I'm trying to make this as pluggable as possible while it's early on.

I also want to borrow the use of a VCR_MODE environment variable that philscatz/fetch-vcr uses to control the runtime behaviour of tests.

• playback: (default) only uses the local fixture files
• cache: tries to use the recorded response and if not found then it is fetched and then saved (useful when adding new tests)
• record: forces HTTP requests and responses are saved to the filesystem (useful for regenerating all the fixtures)

This means one unified implementation, rather than swapping things out on-the-fly in tests.