93_PWMR.pm: keepAlive option

[spaceboy]

Hi there

here is a small patch that adds a keepAlive option to imlement a simple fail-safe operation. From the docs:

When set to 1, the command to turn on the actor will be reissued every 'interval' time if heating in the room should stay on.
This allows for fail-safe setup with actors that have a built-in timer which limits on time (like Eltako FSR14). This way
the actor will turn off the heating in defined time, should something wrong happen to FHEM, the host OS, link, etc.
The 'interval' time should be shorter than run time configured on the actor.

Thanks for review

Petr