Secret environment variable

[ListIndexOutOfRange]

Implements the possibility to use a secret environment variable through the use_secret parameter in the mkdocs.yml file instead of a plain password. This PR applies the changes required after the previous one:

• use_secret is now a string instead of a boolean, allowing for custom name for the environment variable storing the password value.
• the plugin.py class checks for the environment variable availability in order to prevent unwanted data disclosure.
• requirements.txt has been deleted.
• the README has been simplified.

[CoinK0in]

I quickly looked at your PR, it looks really cool :) I find myself a moment to validate that there is no regression, even if I have no doubts and I merge that.

Thank you for the contribution and the extra effort you have made to adapt to my expectations.

[CoinK0in]

It is done. Thanks for the contribution.

Don't hesitate if you have any other idea that could improve this plugin :)

[ListIndexOutOfRange]

Thank you in the first place for this neat plugin ! Yeah, I'll let you know if i think about something ;)