Add dotenv for local development

[pietroalbini]

dotenv allows configuring environment variables loaded by the program in a text file, removing the need to set them manually every time or to add them to a bashrc (or similar). It has no side effects if the .env file is not present.

The use case for it is local development, not production.

[QuietMisdreavus]

I'm generally in favor. We already use environment files in production, but they're loaded via systemd. This allows someone to run the project outside of systemd, without using a weird copy/paste in the bash profile to set the environment for all their sessions. I've been considering putting this configuration in a file of some kind, and this is probably easier than manually parsing some TOML file or the like.

[pietroalbini]

We're going to switch to docker-compose instead of vagrant, so this won't be needed anymore.