Add version / Git commit comment to generated deny.toml

[Marcono1234]

Is your feature request related to a problem? Please describe.
cargo deny init creates a default deny.toml for that version, and it is quite verbose.

The problem is if you later update cargo-deny and also want to (manually) update deny.toml based on the deny.template.toml for that cargo-deny version, it is a bit difficult to find out what you customized originally because you cannot easily see from which template version deny.toml was originally generated.

Describe the solution you'd like
The generated deny.toml (or even deny.template.toml) should have a comment at the top mentioning the cargo-deny version or Git commit for which it was generated.

This way when you manually want to update it later, you can diff with the deny.template.toml file on GitHub from that version to easily see what you had customized.

Even better might be if it included a GitHub URL of deny.template.toml so that you directly know what to compare with. For example:

# Based on https://github.com/EmbarkStudios/cargo-deny/blob/0.16.0/deny.template.toml
# (diff with that version to see what has been manually changed here)

# This template contains all of the possible sections and their default values

# ...

Describe alternatives you've considered
none