Add a way to customize the text input

[olzraiti]

Currently it's impossible to customize the text input that the component wraps. Customizing the text input would allow users to change the type attribute or set the input's id (currently you can't add an id the the react-selectize component and therefore you can't focus the component when a label is clicked). I can think of two ways how the text input could be customized:

1. Add some parameter object like inputOptions which would be passed to the React input element.
2. Add a input rendering function parameter.

Adding a way to customize the input would solve at least #79, #110 #118.

[green2k]

Yeah, it would be also perfect if we're able to set the readOnly property!

https://github.com/furqanZafar/react-selectize/issues/172