Option for more verbose logging - at least the filename loaded, possibly more.

The current logging makes sense: for security the values aren't output by default.

When I'm working on a new project, I'm configuring (initially misconfiguring stuff), I'd like to know the filename that was loaded and ideally the keys and values.

At the moment we get a line number, but because I'm working on a complicated project with a bunch of .env files, I'm left guessing which file is open.

I'd like to be able to set some environment variable and get more info, it's understandable if this doesn't show the keys and values, though at least the filename, and they key would be nice.

Note: I don't have control over where the code is called from so setting from an environment variable would be nice (in this case it's pipenv, as well as a django app I'm working on)

Start by tracing how python-dotenv currently logs loaded files and how environment-variable options are handled. Define the supported verbose output and its security boundaries, then add coverage for selecting that behavior through an environment variable and confirming the loaded filename is reported.