Added VectorLayer type prop and changed layerMixin before prop to String

[luizotcarvalho]

I noticed that prop type on VectorLayer is missing and type fill was set every time (even if was defined as outline), and another problem was that mapboxgl is expecting a string on the before param of addLayer method and was typed as object on layerMixin props

[luizotcarvalho]

And one more thing, the VectorLayer can only be used when url prop was set, but url prop is not required. So I moved alongside optional props when creating the source.

[soal]

I integrated your code manually in version 0.2, so I close this. Thank you for your contribution!