R1:baa9d96082b7

Increase the power of `bin/config`

Summary:
Fixes T2254. Make the CLI for config more powerful:

- Add validation for `set`.
- Add `get`.
- Add `list`.
- Add `delete`.

The `get` command produces fairly verbose JSON to support flags like `--all`, or `--source database` later. The other commands are straightforward.

Test Plan:
Tested `config set`:

$ ./bin/config set
Usage Exception: Specify a configuration key and a value to set it to.
$ ./bin/config set x
Usage Exception: Specify a value…