Use native String#repeat if available.

Also, MUCH faster algorithm otherwise.

String.prototype.repeat made it through to new ECMAScript standard, so it will be bad to overwrite existing methods.

Taken from my es6-shim (https://github.com/paulmillr/es6-shim/blob/master/es6-shim.js).
Fix indentation.
Merge pull request #2 from paulmillr/patch-1

Use native String#repeat if available.