Feature: Allow config to be set using environment variables
Would be handy for these variables so I can keep the nitter.conf file static and shareable for others wanting to reuse my configs:

```
[Server]
hostname = "${...}"

[Config]
hmacKey = "secretkey"
```

Expected mapping would be along the lines of `NITTER_SERVER_HOSTNAME`.

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<summary>Extra question that has since been answered</summary>

To save a issue ticket for a question, can anyone point out where these are used please?

```
[Preferences]
replaceTwitter = "nitter.net"
replaceYouTube = "piped.video"
replaceReddit = "teddit.net"
```

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