Add `-mdc_swipe:` for programmatic swiping.

`-mdc_swipe:` programmatically swipes a view, without requiring a pan
gesture.

- Add `swipeAnimationDuration` and `swipeAnimationOptions` paramters to
  swipe options. These determine the duration and animation options for
  the programmatic swipe animations used in `-mdc_swipe:`.
- Add "like" and "nope" buttons to sample app.
- Add note on `-mdc_swipe:` to README.
- Fix typos in README.
- (internal) Refactor internal methods in `UIView+MDCSwipeToChoose` to
  use state over parameters.
- (internal) Define new type for rotation direction.
Modify internal classes' directory structure.

New directories mirror those of public classes.