f3: head, manpages & test added

Note: There’s a `v4.0` release for f3 but unfortunately it doesn’t compile on OSX. The author is aware of that, and everything will be fixed in the upcoming [`v5.0`][v5] release.

[v5]: https://github.com/AltraMayor/f3/blob/da939f2d7d32d69faa78f2c0d2aea0ac5d5004b2/changelog#L1-L4
Maybe

```rb
man1.install "f3read.1"
man1.install_symlink "f3read.1" => "f3write.1"
```

instead of a copy?