Update demo

Merge pull request #4 from Enrique199413/develop

update demo

Update Doc

Analyze from polymer element

First version implement number-flip

Support Properties

from: The number that animation starts from.
to: The number that animation rolls to.
duration: The animation duration in seconds. If not specified, duration defaults to 1 second.

<flip-number-element> provides the following custom properties
for styling:

Custom property	Description	Default
--flip-number-element-color	Color of the base text	#000
--flip-number-element-style	Mixin applied to the flip	{}