WIP: Allow custom inputs (idea)

[mpourismaiel]

What this PR does / why we need it:
This is an idea I've been thinking about for a while and when I saw the issue, thought I create a PR for it. It's a breaking change since the config will change, but I added default inputs same as our previous sample. Please let me know what you think.

Which issue this PR fixes:
fixes #753

Special notes for your reviewer:
This is a work in progress and an idea. Please do not merge.

Release note:

- **UPDATE IF PR IS APPROVED**

[stp-ip]

This breaks existing users.

How about keeping the default fields and adding the option to add custom ones after the default ones similar to how you set the config:

[[fields.custom]]
  name = "phone"
  text = "Your Phone *"
  error = "Please enter your phone number" # defaults to theme default
  required = true

In the code we would add our default fields and afterwards iterate through the custom ones.

Thoughts?

[mpourismaiel]

That sounds really good.

[stp-ip]

Missed the WIP in the title. The code looked finished so ;)

Let me know, if you want me to revert.

[mpourismaiel]

I wasn't sure if the idea was polished enough. If you think it's good then fantastic!