Add `--list-styles` CLI option

Currently the values that the `-S` (`--style`) option accepts are only discoverable by looking at the soure code:

https://github.com/jgm/skylighting/blob/8b2b85aec695e6618720582f989e47691cb70b68/skylighting/bin/main.hs#L131-L139

I think it would make sense to introduce a `--list-styles` option that would just list the available styles:

```
$ skylighting --list-styles
pygments
espresso
kate
tango
haddock
monochrome
breeze-dark
```

Analogous to `pandoc --list-highlight-styles`. If `--list-styles` is introduced it probably makes sense to rename `--list` to `--list-syntaxes` for consistency.