Support supplying config variables through means other than the main config file
L: I'm thinking of something like environment variables (that override the default config) or a secondary config file (passed via a command line flag, e.g. `--extra-config /etc/nitter_secrets.conf`) here. The reason for this being useful is that the config file may contain secrets (such as the HMAC key), but you might still want to publish the non-secret parts of it. As for environment variables, the mapping might be something like `NITTER_[Section]_[Variable]`, e.g. `NITTER_Config_hmacKey` (though all upper-case, i.e. `NITTER_CONFIG_HMACKEY` would probably be even better). Alternatively, some special placeholder format in the config that makes it get it from an environment variable instead would work, too (e.g.
```
[Config]
hmacKey = "@NITTER_HMAC@"
```
to get it from the `NITTER_HMAC` environment variable).

Edit: Oh, there's also the Redis password, which should be secret too, I guess, and of course there might be other secrets in the future, so uh, it would be best if this was implemented so it could be used for any config variable.