Menu Bar component

The Menu Bar should be a flexible, highly-customizable, all purpose menu bar. Its origins are in the Icon Bar in Foundation 5, but this component will have more options for icon placement and secondary features like labels.

**Features:**
- [x] Horizontal and vertical orientation, with breakpoint classes for both
- [x] Icon positioning (left, top, right, bottom)
  - [x] Breakpoint classes for icon positions
- [x] Spacing options (e.g., left-aligned items, evenly-spaced items)
- [x] Count labels (i.e., unread message count)
- [x] Coloring classes (primary and dark)

**Component requirements:**
- [x] Settings variables
- [x] Layout mixins
- [x] Style mixin