use create-dmg to create a custom macOS disk image

[sweetppro]

Fixes transmission/transmission#4062
Im not sure if this is the correct place to customize the release .dmg, but it was the only obvious location...
Also not sure on how we prepare semantic versioning...
@ckerr @livings124 @mikedld

I have a placeholder custom background, which is easy enough to replace.
for future reference, the dmg_background.png file is 1000px * 620px (144dpi)

[livings124]

This does look really nice but I'm hesitant to clone a third-party repo and then run a command from that in our release scripts

[sweetppro]

This does look really nice but I'm hesitant to clone a third-party repo and then run a command from that in our release scripts

@livings124 that's a good point,
how about we maintain a fork of the repo, and link to our own fork.
then we can at least verify the security of any updates to the repo before we incorporate into the release script.

[sweetppro]

create-dmg has an MIT license