Add basic functionality to use storyboard images on iOS

[holgergp]

Description

This PR adds basic functionality to make use of storyboard images on iOS. Apple will sunset support for exact image sizes on June 30th Link

Related Issue

Closes #51

Motivation and Context

This plugin will not supposedly not work as expected due to Apples new guideline. Please see the link above.

How Has This Been Tested?

I already have this change in use using a fork of this plugin:

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[WuglyakBolgoink]

@devgeeks / @EddyVerbruggen

any roadmap for merging?

cordova-ios@6 supported "only" soryboard. and we see only black/white background instead of storyboard-splashscreen...