Added TypeScript definitions for innerRef.

[cameron-martin]

Closes #424.

What:
Added innerRef prop to styled component typescript definitions.

Why:
To fix this issue: #424

How:
By adding the StyledComponentProps type, which has all the normal element props and also innerRef that is the same type as ref.

Checklist:

• Documentation - N/a
• Tests
• Code complete

I'm not sure what to do about withComponent(AnotherComponent). What even is the behaviour of withComponent when called with another component?

[renatorib]

Hey, this looks great.

But I think StyledComponentProps isn't a good name for this. I think it as something like ElementProps.
I consider that to be a StyledComponentProps it should have all props including Props and Theme

[cameron-martin]

Sure, I renamed it.