Support mode heroic feature toggles

[sjoeboo]

All heroic query "features" can the turned on or off at query time(use a - to turn off a default on feature):

"features": ["com.spotify.heroic.distributed_aggregations","-com.spotify.heroic.shift_range"]

We should support enabling/disabling these in grafana (perhaps under some advances-options section, defaulted to the state heroic has them in by default?)

[sjoeboo]

Features are listed here:

https://spotify.github.io/heroic/docs/config#feature