Allow to add attributes to buttons and spacers

[yvmarques]

This would be useful if we are adding attributes that help us to integrate the HTML with some WYSIWYG (as Mailchimp with mc:edit). Most of the existing components keep the attributes, but not the button.

I think the right place to put the attributes should be the element, as everything surrounding the link is just a helper for the link to show nicely.

[yvmarques]

I will try to add one test if I got time :)

[rafibomb]

Thanks for the PR - is this what you're looking for? #42

[yvmarques]

I am looking for more flexible solution for specific attributes that some 3rd services might use for their WYSIWYG, like for example, Mailchimp uses mc:edit to allow the users edit some portion of the text.

The same for CampaignMonitor uses cm_dontconvertlink to don't track links.

This PR allows we add as many attributes as we might need.

[cossssmin]

I was going to submit a PR, and then saw this - awesome!

It's currently the only thing preventing developing templates with custom attributes (like ESP specific) in Inky. I'm currently duplicating files from dist/ and working on the compiled HTML - it's a huge pain, especially when you need to update a page/partial with Inky: you need to start over with integrating the custom attributes...

Any planned release date? :)

[rafibomb]

Awesome - merged! Thanks!