Add stateSelector.js in Element.
- Add State attribute: Disabled, Focus, Active, Loading, Error, Completed.
- Each elements can support this attribute.

Signed-off-by: Lee <jessy1092@gmail.com>
v0.1.0
- Add State attribute: Disabled, Focus, Active, Loading, Error, Completed.
- Below elements can support thease attributes.
  + Button: `active`, `disable`, `loading`
  + Header: `disable`
  + Icon: `disable`, `loading`
  + Image: `disable`
  + Input: `loading`, `focus`, 'error'
  + Loader: `disable`
  + Reveal: `disable`
  + Segment: `disable`, `loading`
  + Step: `active`, `disable`, `completed`

Signed-off-by: Lee <jessy1092@gmail.com>