Add initialSelectedIndex property to Tabs

[WRidder]

Fixes #375. Also checks whether the index is not out of bounds w.r.t. the amount of tab children.

[hai-cea]

@WRidder Can you please also add PropType validation as well?

initialSelectedIndex: React.PropTypes.number

Also, please add the prop to the docs here:
https://github.com/callemall/material-ui/blob/master/docs/src/app/components/pages/components/tabs.jsx#L58

Thanks!

[WRidder]

Alright, updated proptype validation. I also added the proptype validation for tabWidth since it was specified in the docs already. Added initialSelectedIndex to the docs as well.

[hai-cea]

Thanks @WRidder 👍

[ferrannp]

Uf thanks for this one, very convenient specially for routing when sending a direct link to somebody. Any idea when this will be released?

[mmrtnz]

Hopefully we can get v0.7.2 ironed out by Wednesday at the latest.