This commit introduces advanced configuration. The rate-interval and
label-prefix flags are removed, and replaced by a configuration file
that allows you to specify series queries and the rules for transforming
those into metrics queries and API resources.
The `build` Makefile target attempted to place the generated artifact in
`/build`, where it should have used a local directory instead. This
commit makes the directory configurable using the `OUT_DIR` argument,
and defaults it to ./_output
The initial functionality works. There's still a number of TODOs to
clean up, and some edge cases to work around, and some errors that could
be handled better.
