Provide keyboard shortcuts to change settings "on the fly"

[yarikoptic]

I am particularly interested in adjusting threshold and mask_update_speed to potentially (didn't play with much yet) adjust masking of my precious myself masking -- by default it is too "ghostly" ATM. And potentially print out adjusted values so people could then incorporate them into config files (or CLI invocation)

[yarikoptic]

FWIW, for my use case I had to set mask_update_speed to 99 to get some reasonable effect... changing to 99.999 seems to be of no effect, so either it gets intified or not "percents"?

[fangfufu]

Feel free to submit a patch for this.