Being able to specify configurations with config file

Right now it's possible to specify runtime options thru flags and environment variables. Could it be possible to use a config file instead?

The reasoning behind this would be to use `vault-agent` or other configuration management tools separate from the binary

I'd imagine something like `influxd --config=path/to/config.yml`

Start by tracing how influxd currently reads runtime options from flags and environment variables. Define the supported config-file format and option mapping, then verify that a path such as influxd --config=path/to/config.yml can provide the same runtime configuration.

Written by the indexing model from the issue text.