Add support for text input tracking

[mlmitch]

I want the sensor to generate events that track user text input on websites they visit.
Attacks (e.g. credential phishing) happen through text input.
Also some other security hygiene / trends might be visible from this data.

Challenges:

• Sensitive data encountered here (e.g. password fields). Need a method of dealing with this.
• Large pieces of data (e.g. online documents) encountered. Need a method of dealing with this.
• Possibly many pieces of data. Only want to send what's valuable to the gateway.
• Many ways of entering text. Should work with things like autofill, password managers, copy/paste, keyboard