Add margin support to Image

[danielost]

Added margin support to Image (Issue #122)

• Tested it on several cases, and it works as expected. One of the test cases was the rollingCircles from the Doodle Documentation (end of the Layout section);
• Overloaded margin method - it allows more flexibility in specifying margins by accepting 1, 2, or 4 arguments (just like in Picture).

Here is the example itself, but with the Picture replaced by Image:

val circle = Image.circle(50)
val rollingCircles =
  circle
    .margin(25)
    .debug
    .beside(circle.margin(15).debug)
    .beside(circle.debug)
    .beside(circle.margin(-15).debug)
    .beside(circle.margin(-25).debug)

And here is the result:

This is my very first PR, so please let me know if I did something in the wrong way

[noelwelsh]

Thanks for doing this! Looks great.

CI has been tripped up by formatting. I'll merge as is and fix it up myself, and update the README with instructions on running formatting etc.

[danielost]

Glad to help! And thanks for updating the instructions.