Time Proportioned Output support

This extension adds a Time Proportioned Digital Output feature into the Tasmota software.

The relay output on a Sonoff device provides (obviously) just on/off control. Often it is desirable to be able to get a value between off and on, for example 25% power. The conventional way to achieve this with devices such as electrical heaters or hot water radiators is to switch the device on for a period and then off for a period. This extension allows a required power value between 0 and 1 to be specified via MQTT and the code will automatically cycle the relay on/off to achieve this power. The s/w is configured with a Cycle Time that specifies the period. So for example if a power value of 0.25 is specified with a cycle time of 12 minutes then the relay will be on for 3 minutes and off for 9 minutes every 12 minute period.

The s/w includes a safety mechanism where the maximum time expected between MQTT power updates is specified. If this time is exceeded with no value being received then the power will revert to a specified fallback value.

Instructions for setting it up and using it are in xdrv_48_timeprop.ino. The feature adds about 1.2k to the compiled code.

Currently all configuration parameters must be setup at build time. If anyone wanted to add these to the MQTT and/or web interfaces a PR would be gratefully received.