add anchorProps to <Link> so that attributes can be set to the enclosed anchor element

[dpnolte]

Hi, First of all, thank you for the awesome package.

This PR is completely based on https://github.com/react-navigation/web/pull/12, so full credits to @clemMoine . I needed to be able to set attributes to the anchor attributes (styling, data-, onclick, etc.), and created my own fork based on the feedback in #12 from @ericvicenti . However, that PR has been inactive for a while. That's why I'm opening this PR in the hope it gets fixed sooner. @clemMoine, if you rather want to update your PR, I'll delete this one.

Exmaple Usage:

                <Link
                  routeName="Docs"
                  navigation={navigation}
                  anchorProps={{
                    'data-testid': 'docs-link',
                    style: {
                       color: 'silver',
                    },
                  }}
                >
                  Docs
                </Link>

Best regards, Derk