Add some configuration options

[laynor]

Hi, first of all, thanks for the work you and the original author put on this package!

To satisfy my company needs, I added some configuration options to the Crop widget:

• fixedCropArea
  The crop area is shown in its default position and cannot be moved or resized.
• ovalCropArea
  The crop area is shown as an oval. Please note this has no effect on the resulting cropped image, it's just aesthetic.
• showHandles
  Whether or not to show the resize handles - I believe this is useful only to hide the handles when using a fixed crop area.
  In our case, we use it when we're cropping an avatar image, as we have a fixed, oval crop area.
• style
  The constants used to define colors, sizes and durations in src/crop.dart have been collected into a CropStyle class so that it is possible to customize them.

Please let me know if these changes are acceptable for you.