Add anti-overlap feature

Based on PR #14 by Coobaha. Changed examples and tests to use kwargs,
cleaned up the documentation, and slightly refactored the `between` mixin.
Refactor anti-overlap feature and merge in scoped variables

Changes:
- Merges with the scoped variables changes.
- `rupture.anti-overlap` variable now accepts booleans or single values, and can covert
  between pixel and em values based on rupture.base-font-size.
- `enable-anti-overlap` variable removed.
- `force-anti-overlap` argument renamed to `anti-overlap`, and now accepts the same
  values as `rupture.anti-overlap`.
- Fixes a bug / typo in the anti-overlap test.
- Update readme.
Merge pull request #22 from sbking/anti-overlap

Add anti-overlap feature w/ kwarg examples