initial progress towards graphite support

not meant for merging, but just for discussion, feedback, etc.
this just parses `graphite("graphite-expr")` in the config. right now there's only 1 arg, the expression.
i presume i should a second arg to have a `from=-2minutes`, which i presume is also what the `sduration` ("2m") does for opentsdb.

next step: actually having this query graphite, figuring out in which shape i should feed the timeseries data from graphite into bosun, and figuring out whether or not it's a problem that there's no tags, ever, when using graphite (empty map of tags).
something that graphite does is if you query for "foo.bar.*" and it returns a series "foo.bar.web1", "foo.bar.web2", "foo.bar.web3" i could probably somehow automatically compute tags node3=web1, node3=web2, node3=web3, but i'm not sure if this is needed, and it might be much effort if i need to add "tag awareness" in several places i didn't expect.