Make RTL support configurable

[bryankeller]

Details

https://github.com/airbnb/MagazineLayout/pull/30 added RTL layout support, but did not make it configurable. To enable more control, I've added an initializer to control flipsHorizontallyInOppositeLayoutDirection, with a default of true.

Related Issue

N/A

Motivation and Context

This makes RTL support opt-out. Currently, it's not configurable at all.

How Has This Been Tested

Tested RTL support in the sample app.

Types of changes

• [ ] Docs change / refactoring / dependency upgrade
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.