Implements: https://todo.sr.ht/~witcher/rss-email/20

Implements: https://todo.sr.ht/~witcher/rss-email/22

With simple_logger the logging level can bet set with an argument instead of relying on an environment variable. This enables the command line flag `-v` for specifying the logging level, which is a little more ergonomic than an environment variable.

All possible configuration file attributes can be given on the command line instead and, if all have been given, no configuration file is needed for the program to run. Implements: https://todo.sr.ht/~witcher/rss-email/24 References: https://todo.sr.ht/~witcher/rss-email/13

The configuration attributes `to` and `from` from the global section of the configuration file have been moved into a `mail` section. References: https://todo.sr.ht/~witcher/rss-email/24

The `Mail` struct allows the caller to not have to worry about the format of an email.

Also fix `sqlx-cli` mistakenly being referred to as `cargo-sqlx`.

Implements: https://todo.sr.ht/~witcher/rss-email/18

Lines in the URLs file starting with `#` are filtered out. Implements: https://todo.sr.ht/~witcher/rss-email/17

I had previously implemented sending of emails with the async transport (together with async fetching of RSS), though I now realise I'd never sent the patch. While implementing this, I realised that the "pool" feature has a huge impact on the speed of sending emails: by merely having the feature enabled the transport instance will keep the connection open between messages (avoiding a connecting and authenticating a second time). Just leaving a comment on that for future reference.

Release notes can be found at https://git.sr.ht/~witcher/rss-email/refs/0.3.0

Thanks to the introduction of tokio in the codebase, E-Mails can now be sent asynchronously as well, speeding up the implementation even more. Implements: https://todo.sr.ht/~witcher/rss-email/16