[gatsby-link] Added handling of replace property.

[tomsseisums]

Reach Router has it.
React Router has it.
Probably any other router out there has it too.

A need arose for me to have such functionality, was surprised it doesn't exist, so, I made one.
The need for me is opening/closing modals, not really looking for them to be stored in history.

I'm not sure if this won't have to be changed once reach/router#119 lands, but in the same fashion as scroll behavior has been fixed for now, made the quick-fix here.

[pieh]

Code looks good - could you document this prop in https://github.com/gatsbyjs/gatsby/blob/master/docs/docs/gatsby-link.md ? It actually would be great, if we would have list of props there (noting which ones are required), but just adding something about replace would be enough

[tomsseisums]

Yeah, will do.
Forgot about docs. :(

[DSchau]

Looks good to me!

[gatsbot]

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