Add Servo support

[zafrirron]

Nice addition to Espurna is mechanical servo (pwm step motors) support... I've implemented Servo support (as a 3'rd party plugin to begin with),

1. Using Relay switches to set predefined positions
2. Using Button to move in predefined steps
3. Saving manual position, recall saved positions
4. API integration (terminal/rest) to set and get Servo position
5. Using set/get settings to save last position . . and all other espurna goodies...

If this is an interesting use case, this could easily become a new "relay" type, to support multiple Servos as native espurna thing...

Using: Servo library core esp8266 waveform generator

[mcspr]

If this works for you, sure.

I have only minor questions:

• have you tried our internal pwm implementation? see lights dimmer setup, libs/pwm.h & pwm.c for the API.
• do we need to include any external stuff or do we only depend on the Core + some wiring in our modules?