Feature: ionic resources source different images for landscape and portrait

As a feature request it would be nice if we could use a different source image for landscape and portrait.

Right now the tool crops the source image to fit for landscape and portrait, however this doesn't always produce best results with images expected to take up the full screen.

The same method of cropping can be used on different source images. This will allow the image to take up the full screen in both landscape and portrait without having black bars in one mode (or half the image cut off)

Example use case:
Portrait: An image of a tall building.
Landscape: An image of a city skyline including the tall building

Filenaming:
Currently we use splash.png.
For this feature we could have 2 files, splash-landscape.png and splash-portrait.png.