Fix bug when empty flipId array throws an error

Address #76 by returning out of animateFlippedElements function early if there are no flipIds present (and fallback to always selecting body as the parent if there is no firstElement found).

Fix edge case bug when layout changes based on transform attributes

Previously, the library did not apply the identity transform as an inline style, assuming that it was redundant. This breaks FLIP (by creating a single frame flicker) for certain layout animations that are based on changes in the transform value.

Add onComplete callback to Flipper and onSpringUpdate callback to Flipped

• onComplete added to Flipper for a way to have a callback attached when all FLIP animations are complete
• onSpringUpdate added to Flipped for a way to call a callback continuously with the current spring value for the associated Flipped component. This might be helpful to choreograph complex animations that have both FLIP and non-FLIP components.

Fix iframe bug

Issue #73

Add decisionData to core.ts

Merging in this pr

Add shouldInvert function to core

#68

Make proptypes a peer dependency

The library was getting imported into the umd bundle, increasing its size

Add back callback capability to inverted Flipped components

Fixes issue #50 and fully removes identity transform to close out #51

add the onStartImmediate function

Addresses the problem found by issue #46

Bugfix

Fix for issue #44