Make Fallback() optional

[zeisss]

Hi,

I just stumbled upon the original netflix article and was searching, whether someone did something similar for Go. And there you are :) Thanks!

I was wondering, why you made the Fallback() function required? I think one could just define an aditional interface, move the fallback function there and check (on error in Run()), if the command can be converted to the Fallback interface.

Just a thought I had while reading your README (I haven't actually tried anything yet).

Thanks again!

P.S.: The same could be done for the Timeout. Just default to something like 5 seconds.

[dahernan]

I'll have a look to your PR tomorrow, I was thinking in implementing that