Feature Proposal: Form content filters

## The Issue:
In the past view days I got a **lot** of spam through a contact form on my website. I get about three spam mails a day.

## How to fix?
It would be great if you could manage filters in the Plugin settings for Emails and form content. If I had this filtering available I could enter the key words that are in all of those spam mails to prevent future messages. All of those spam mails contain the keywords `cashnow.cars`, `Loan` and `Payday`.

## How to implement?
It would be great if you can configure a set of filters in the plugin settings page and load them into each form separately. Like:

```yaml
form:
filters: filter1, filter2
name: contact-form
...
```

## Is this even possible?
Yes it is. But its just a matter of time to get it done as it requires to create a new settings page to manage the filters, adding the parsing of the `filters` tag within the forms as well as making the filtering system itself.

----
In the case you have more questions about what I mean or how I would like it to be and I don't respond to your massage on Github, contact me at admin.atjontv@atvg-studios.at and inform me about your message.

PS: Thanks to every dev at Grav for reading my proposal!

Start by inspecting the plugin settings page, the form parser that would handle the `filters` tag, and the existing form validation or filtering entry points. Compare the proposed YAML configuration with current form configuration, then define the settings, filter assignment, and spam-blocking behavior before implementation; done means configured keywords can prevent matching form messages.

Written by the indexing model from the issue text.