Add state to Creatable to avoid mutating the options prop

[josh--newman]

Creatable was mutating it's own options prop and throwing a TypeError. The best reason for this that I could find was that since React v14 they freeze props: https://facebook.github.io/react/blog/2015/10/07/react-v0.14.html#breaking-changes.

Funny thing is I couldn't see anybody on the upstream react-select project complaining about this.

Since this is blocking me from using this component I went ahead and fixed it myself here.

[coveralls]

Coverage decreased (-0.006%) to 88.72% when pulling 84a0466 on bugfix/dont-mutate-props into 405b575 on master.