Pull dynamic data from website fields

[GaurangTandon]

Feature suggested by multiple users on both support email and the support page

Dependencies: #131, #132

Use case:

"A person who uses LinkedIn Messaging (or any comms platform) a lot would like to insert a snippet that automatically enters the name of the person (or any other related dynamic data related to it) that he's communicating with. For example:

Hi %name%, I hope you are well. Let's schedule a call sometime soon. What's your availability next week? Regards, James

He doesn't want to have to type the person's name because he might misspell it, potentially ruining a relationship. Plus, typing the name slows him down. "

The name and related data are stored in an element indicated by the user using a selector sel.

Todo:

1. Design a syntax for form fill in which supports this.
2. Pull out data from webpage based on selector and paste it inside snippet body.

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Related: #34 #167