Add dotenv-override setting

[Lun4m]

Closes #733

It's maybe too naive? I added some basic tests, but can you think of other edge cases I should test?
I also need to add some documentation I guess.

[mikeyhew]

@Lun4m What's your use case for this feature? I don't see a comment from you in the linked issue.

This seems compatible with #2682 but there will be merge conflicts and it will be important we have good test cases for both features

For what it's worth, if this config option does get added, I think dotenv-takes-precedence would be a clearer name.

[casey]

Seems reasonable to me! I notice that the original issue has eight thumbs up, so it seems like something people want.

I made some changes:

• Added documentation to the readme
• Passed in the whole Settings file instead of just the bool. I think this is slightly less error prone, since it avoids a bug where you pass in the wrong bool to a function.
• Made it so that if dotenv-override is set, environment file loading is trigger, since this is the behavior of existing environment file settings