Preliminary PR: ProMotion 1.2

Since the [discussion around ProMotion 2.0](https://github.com/clearsightstudio/ProMotion/issues/356) is taking ProMotion in a new direction, we'd like to release a version 1.2.

This release will include some features that were originally slated for PM 2.0; namely, use of `motion_require`.

This preliminary pull request does not address deprecations. I hope to add deprecation warnings soon, perhaps in a lighter 1.3 release.

From now on, `edge` will refer to 1.x releases and something like `pm2` will refer to the 2.0 branch.

### Changelog

1. Now uses [motion-require](https://github.com/clayallsopp/motion-require) for better compatibility with other libraries https://github.com/clearsightstudio/ProMotion/commit/6046dd7f49ba174f309baaa428eaac80ce0290b6
2. SplitScreen: New option `swipe:` allows you to specify if a splitscreen master screen can be opened with a swipe. https://github.com/clearsightstudio/ProMotion/commit/ffbb76caf3071297347d4cf43784069999a746b3
3. FormotionScreen: Removed `PM::FormotionScreen` and the Formotion testing dependency. Use [ProMotion-formotion](https://github.com/rheoli/ProMotion-formotion) instead.
4. Styling: Added a `content_width` helper to PM::Styling (similar to `content_height`) https://github.com/clearsightstudio/ProMotion/commit/08a984815a7c96b9465c31b2e2664ac0086d2e1c
5. TableScreen: Removed SDWebImage in favor of similar but more reliable JDImageCache https://github.com/clearsightstudio/ProMotion/commit/59ed747e93567e32bdb5099fed12297161cea05a
6. Screen: Allow custom views (including images) for `title` setting https://github.com/clearsightstudio/ProMotion/pull/415