Text styles, multiple classes + webpack-plugin improvements

• Add more elements to permitted text styles
  • <small>, <text>, <label>, <legend>, <svg>, <code>
• Allow {N} number of multiple classes
• Transform relative paths to absolute in webpack-plugin

Improved multiple classes and test coverage

• Improve usability of multiple classes (see issue #4). This change is backwards compatible, which means multiple classes can be separated with a query selector (for versions < 1.0.51) and with a space (for versions >= 1.0.51).
• Test coverage for multiple classes
• Test coverage for immutable mixins

Fixing modifier classes

• Fix generated CSS selector for modifier classes 🐛