gatsby-remark-images supports setting position of figcaption

[Cygra]

Currently gatsby-remark-images generates a figcaption tag which defaults to the left side of the image.
It would be better to support a position option:

Name	Default	Description
position	left	Postion of the caption: left | middle | right

I would like to submit a PR for this feature if approved.

Caption on left:

Caption at middle: