[Bugfix] Animate progress bar on mount, update demo

[lsanwick]

Builds on top of #21

The progress bar would not animate on the initial mount of the component because no CSS transition would occur (the bar starts at the specified width). This adds handling to start the bar at 0 and transition to the passed in completed value.

I've also updated the code for the demo to work correctly and added a script to build the demo, build-demo. I don't know how this gets published to GitHub Pages, but that would need to be done after rebuilding the demo code.

[coveralls]

Coverage remained the same at 100.0% when pulling 1c747e4 on lsanwick:ls/animation-fix into 4d6928a on abdennour:master.

[lsanwick]

@abdennour any chance you can review this?

[abdennour]

Hello @lsanwick sorry for the delay in getting back to you

[lsanwick]

No worries, thanks for taking a look!

[lsanwick]

@abdennour Can you look at this now? I've rebased it against the current master branch. Not sure how the demo gets published, so that build product might need to be ignored.