Add power on value setting to switch

[OttoWinter]

Description:

Using inverted is too much of a hack.

Pull request in esphomedocs with documentation (if applicable): OttoWinter/esphomedocs#TODO
Pull request in esphomeyaml with YAML changes (if applicable): esphome/esphome#180

Example entry for YAML configuration (if applicable):

Checklist:

• The code change is tested and works locally.
• Check this box if you have read, understand, comply, and agree with the Code of Conduct.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphomedocs.