Added ability to close to previous screen

[jamonholmgren]

So, @GantMan and @darinwilson and @leonskim and I worked on this one for quite a while. The upshot is that you need to provide a custom activity if you want to use close to_screen.

class MyActivity < PMSingleFragmentActivity
end

# ...
open MyScreen, activity: MyActivity

# ...
close to_screen: MyScreen, activity: MyActivity

If you don't provide an activity both when you open the screen and when you close to it, it won't work.

[GantMan]

Dude you got it!? Awesome!

[jamonholmgren]

Yeah, it works!

[darinwilson]

Nice work!

[jamonholmgren]

Mergeable?