add support for outbound link click tracking #4

[mehul0810]

This PR resolves #4

@metmarkosaric I have implemented this functionality using a custom snippet (minified) based on code here: https://github.com/plausible/analytics/blob/1504dd5412e2aad006c7ec669c342c649b1da186/tracker/src/plausible.js#L59-L76

I have tested this and is working as expected. But, I want you to take a look on this PR as well and merge the PR if it looks good to you.

Visuals

Breakdown: URL

Breakdown: Referrer

I think if we agree to go with custom snippet for Outbound links then we can add it as a default goal for all new sites added to plausible.io.

[metmarkosaric]

nice, thanks @mehul0810! i like that! we still have many customers who are not using WordPress so we have to consider them too on plausible.io. i'll let you know if the default goals can be set when i speak to Uku. but even a quick visit to plausible.io to set up these two is not too bad as you've integrated the biggest part of the work into the plugin already.