Support multiple thumbs

[danielgindi]

Now we can pass an array to value to achieve multiple thumbs. thumbImage accepts an array also - to override individual thumbs. There's a new thumbView property, that works like thumbImage but supplies the whole thumb View.

An idea I took from @doctadre's PR, is better naming for the following props: minimumTrackTintColor is deprecated, use trackHighlightColor maximumTrackTintColor is deprecated, use trackColor

[StevenMasini]

@danielgindi This feature would be great, where are we with it ?

[danielgindi]

It's just waiting for a merge!

[StevenMasini]

@idealllee @RoshOne4 Can it be merged ?

[devth]

@jeanregisser can we get this merged? Awesome feature.