Adding functionality to set frame index directly

[liuloppan]

• Adding functionality to set the intital "current frame index" by passing it as a prop into Carousel.
• Adding the function setFrame to Carousel and passing it as a Widget prop ('setFrameHandler') which can be used by Indicator dots to transition multiple frames at once.

[liuloppan]

Solves issues: #125 #100

[amio]

Thanks @liuloppan! will look into it soon :D

[liuloppan]

@liuloppan Great! Just a few changes:

• since loop cannot work with currentFrameIndex, we should log a warning in case of that,
• let's keep the default demo as original, add some description of currentFrameIndex in README.md

• loop works with my intended use of currentFrameIndex, I renamed it to onStartFrameIndex and added description in README.md
• I changed back to the original demo index.js