Make optional props actually optional in typescript

[mayorandrew]

Hi there! Thanks for the nice and concise library.
I've been using it in my typescript project and found that some of the optional props are marked as required in the typescript IProps type. So here is the small PR which makes them optional so my JetBrains IDEA won't complain about me not passing them

[mayorandrew]

Hmm, turns out it's more complicated than simply marking them as optional. Because then typescript complains when the library is built. Going to research that a little bit

[mayorandrew]

It turns out it is handled by typescript 3.0+ automatically. And my issue with IDE complaining about types is related to the fact that I am directly extending IProps in my component which wraps Range. Seems like I should use custom type for prop type inference for my component, like described here typescript-cheatsheets/react#61 (comment)

Goint to close that PR then, sorry for bothering