Add support for Multiple Tags

@bobpotter, thanks for your work on this. I've had a quick look at the code. Do you think you could summarise what you've done? Did you follow the approach of firfin, or a-fro, or something else?

I'm setting the status to Needs Review, to alert people to the need to check what's been done.

The commits being done here look like to be more related to 💬 Add Tag(s) to signup form Fixed than this issue.

It looks like the work that has been committed so far came from #8, was supplemented by 🐛 Update mailchimp_update_member_process for $tags Active , and also adds a text field to the signup form where tags can be typed in. This partially fulfills the request but I am having trouble understanding the request well enough to know what else is needed and if it would be useful to users of this module in general.