Add decimal separator prop

[ghost]

This will add decimal separator prop.
A thing to notice is the parser prop doesn't adjust to this prop, this allows the custom separator as well as a dot to be used for input but the displaying value will show the custom separator.
I guess people will just have to pass a parser prop if they don't want a dot to be used?

[coveralls]

Coverage decreased (-0.4%) to 84.342% when pulling 9ad716e on erwin-k:master into a2383c2 on react-component:master.

[zombieJ]

ref: #114