add support for 404 error pages tracking #6

[mehul0810]

This PR resolves #6

This PR also includes support for #4 and #3. But, I will test those issues with develop branch again and then will update accordingly.

Visuals

@metmarkosaric I assume that we should keep 404 tracking functionality enabled by default so I have not provided any admin settings to disable it as tracking custom events and 404 will be essential need these days. But, I have provided a WP filter to disable it.

Can you please review the PR and merge if it looks good?

Suggestion

@metmarkosaric I have a suggestion that if we include some default goals whenever a user add a site on plausible.io then it will be plug'n'play functionality for users using the plugin. See screenshot for what screen I am talking about.

For example, 404 goal will be automatically added when we add a site. So, installing this plugin will be enough without any configuration.

What do you think?

[metmarkosaric]

thanks a lot @mehul0810! it's good to keep it all enabled by default. about the suggestion to have few automatic goals in plausible.io when users add a new site. 404 would be easier as it is our default script but the outbound link click tracking is a different script so it's something i'll discuss with Uku and see how it could work.