Automatically start a poller when telemetry starts

[josevalim]

It should be configurable like this:

config :telemetry_poller,
  vm_measurements: :default, # :default | [:foo, :bar] | :all
  period: ...

The default values for this poller are:

Keyword.merge([name: Telemetry.Poller, vm_measurements: :default], Application.get_all_env(:telemetry_poller))

We should change start_link to have the vm_measurements function so we unify the APIs. The vm_measuremets function that exists should be removed.