Add logic in Output for minimum power setting

[mtl010957]

Description:

Adding a min_power configuration to the output component, complementary to the max_power configuration already present. This setting will let the user clamp the output of any floating point variable component like ESP8266 PWM to make the full range between min and max available to the frontend in HA. Servos especially will benefit from this, since many standard servos operate in a range between 10%-20% duty cycle.

Related issue (if applicable): fixes feature_requests/#64

Pull request in esphome with python changes (if applicable): esphome/esphome#
Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Checklist:

• The code change is tested and works locally.
• The code change follows the standards

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

• Documentation added/updated in esphome-docs.

[OttoWinter]

LGTM, thanks 👍 One comment in docs repo