Allow Attributes in HTML Form

[dandrzejewski]

Describe the solution you'd like
I would like the ability to add custom inputs to the HTML forms to capture additional information about users that will be stored as attributes.

For example, if I added:

<p><input type="text" name="topping" placeholder="Favorite Pizza Topping" /></p>

to an HTML form, I'd like it to store the attribute topping for that subscriber.

For that matter, it might be nice to be able to include these attributes on the built-in form at /subscription/form.

[srikary12]

I'm willing to work on this :)

[knadh]

@dandrzejewski please refer to this: #105

[dandrzejewski]

@dandrzejewski please refer to this: #105

Yeah.... I was kind of hoping for a no-code solution to this, but I understand.