Allow displayOption function to return an element

[vilppuvuorinen]

Relatively small changes to allow using custom element via displayOption prop inside the result list. The entire element will still be rendered inside <a href="#"> ... </a>, but otherwise anything goes. Additionally, displayOption could no longer generate a string representation of the given option object without help. Summary of changes:

• Option allows elements as children
• Typeahead's _onOptionSelected converts optionString into a string toString if given value is not a string

Unit tests are included.

[vilppuvuorinen]

Looks like there is also a bit more invasive alternative #152.

[gregeinfrank]

@fmoo any plans to support this?