Add --list-styles CLI option

[not-my-profile]

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

skylighting/skylighting/bin/main.hs

Lines 131 to 139 in 8b2b85a

	styleOf (Sty s : _) = case map toLower s of
	"pygments" -> return pygments
	"espresso" -> return espresso
	"kate" -> return kate
	"tango" -> return tango
	"haddock" -> return haddock
	"monochrome" -> return monochrome
	"breeze-dark" -> return breezeDark
	_ -> err $ "Unknown style: " ++ s

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.

[jgm]

Sure. Not a high priority for me, but I'd accept a PR.