styling: Dividers should match Material Design spec.

[rbutera]

Dividers Specification

Places where need to fix:

• [x] item-divider

Properties that need fixing:

• [x] color
• [ ] padding
• [ ] sub-dividers
• [x] first child dividers

Considerations:

• [ ] light themes?

[rbutera]

in 0.3.0 I've:

• removed border from first-child dividers
• increased divider tile height
• changed divider border color to match spec (uses opacity)

still got more to do so leaving this issue open