Subscriber custom fields

[fulldecent]

Hello, this software looks great and glad to see this shared by the Mettle team.

Can we please consider to add per-subscriber profile fields?

1. Arbitrary fields can be created, any name, maybe some data type restrictions
2. These will apply to the subscriber list (and maybe inside a "workspace" not sure how workspaces work)
3. Can set each field for each subscriber
4. Templates can use any field using the same {{FIELD}} syntax

And then the obvious feature:

1. Templates support {{if}} {{else}} syntax to selectively include content. Or templates supports full handlebars/mustache syntax for template language.

[carallel]

Agreed - this would be great!