Add support for imagemagick stacks

[Sebmaster]

This seemed like a convenient API suggested by @boutell. I previously also had a draft for an API like

gm('img').batch([
 gm({ clone: 0 }).resize(100, 100).out('-write', 'test.png')
 gm({ clone: 0 }).resize(100, 100).out('-write', 'test2.png')
])

but that seemed much more inconvenient.

Fixes #204.