add boomerang and duration flags

[bnlcas]

Pull request to add a way to only zoom in one direction and to control the length/speed (Feature Request).

• Duration is changed by adding a flag for the duration in seconds and adjusting the number of steps accordingly.

• Boomerang Zoom is toggled by a flag to set the value of 0/1 and control the concatenation of the frames in reverse.

Happy to edit the api on this.
Also, if it helps it might make sense to also add a --help flag to explain the options on autozoom.py

[m-m-0-m]

You may want to update this commit to be compatible with the recent update (e629354).