[Feature request] (don't) use filename as fallback username setting

[leogott]

Please add a toggle switch in the extension settings for whether to use secret's filename as fallback username, and consider setting it to false by default.

Reasoning: My (and gopasses) security model considers filenames leaky, so I don't plan to put (parts of) my credentials in the filename.
When I encounter this feature, it's only when I use (imported) secrets, where the 'user' field is named differently.