tor: additional optional deps

An additional optional dep for Tor.

Also removes the example file, as upstream regenerate it periodically and currently we don’t seem to respect that regeneration and leave outdated sample files in etc.

Adds a set of caveats about the ` torrc ` file, similar to the ones we have for ` clamav ` and others.
Nope. Not a bottle thing. There's something weird going on.

```
touch /usr/local/etc/tor/torrc.sample
cat /usr/local/etc/tor/torrc.sample
{blank response}
brew install -v tor
🍺  /usr/local/Cellar/tor/0.2.5.10: 19 files, 6.2M
cat /usr/local/etc/tor/torrc.sample
{blank response}
```

The bottles *do* contain ` a ` torrc.sample ` file in ` etc ` but for some reason, if one already exists, it's being written as ` torrc.sample.default ` instead. 

A build from source *does* overwrite that ` torrc.sample ` file in favour of the newer versions ( ``` /usr/bin/install -c -m 644 src/config/torrc.sample '/usr/local/etc/tor' ``` ), but the bottles don't, despite containing the correct file in the correct places. If there's no ` torrc.sample ` file already, the bottles behave the same way as the source install does.