Added --croppingminimum option to set a cropping minimum area ratio

[fbriere]

Closes #342

[fbriere]

I defined the command-line argument as a simple ratio (up to 1.0), but maybe a percentage (up to 100.0) would have been more intuitive? Feedback welcome.

[clach04]

cropping power appears to have same scale so seems reasonable to me :+1:

[fbriere]

It just struck me that there's no need to actually choose between the two: both could be supported at the same time, allowing for either 0.85 or 85 as an example. (In the unlikely situation where you want a minimum of 1% or less, then you would be required to use a ratio as originally planned.)

[darodi]

@fbriere thanks merged in v5.6.1