Make it easier to switch to another database backend

[burrscurr]

Hi! I personally use a mail server setup created from Thomas Leistner's tutorial and am very happy with it. I use a postgres database instead of a mysql one.

To make it possible to use this management tool, I made some changes that help to switch to another database backend like postgres. Now, the database backend is specified in a single line and can be switched more easily.

I'm pretty new to rust, so I hope my changes are fine. This PR should not change the behaviour of vmail-lib.

[awidegreen]

Thank you.

Are you planning to take the changes a step further and add Postgres support to vmail?

[burrscurr]

I think it would be nice to support postgres. Currently, one has to change the cargo file and two lines in the code to use a postgres database.

I see these options:

• add postgres besides mysql to Cargo.toml and show example in README.md how to modify vmail-lib/src/database.rs to support postgres databases
• Add some kind of build option that allows using a postgres backend
• Configure the database backend (or even, derive it) from the DATABASE_URL value of the env file

I like the second, and even more, the third approach. Unfortunately, I don't know how exactly build options could be implemented, and for the implicit env-file option I'm not really sure if it is possible in rust to set the type of the database connection in runtime.

In general, I enjoy working with rust very much, so I'd be happy to add postgres support and learn more about the language.

[burrscurr]

Well, build options were documented well enough, so I created a PR for the second option implementing just this.