[gatsby-plugin-google-analytics] Extends OutboundLink onClick event instead of override it

[valse]

As subject extends the onClick event defined in the OutboundLink component, for example for logging purpose.

<OutboundLink
  href="https://www.gatsbyjs.org/packages/gatsby-plugin-google-analytics/"
  onClick={this.log}
>
  Visit the Google Analytics plugin page!
</OutboundLink>

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[m-allanson]

Thanks @valse

[KyleAMathews]

Thanks 👍