Document `cabal-fmt` comment pragmas

[elldritch]

It would be great if these were documented in README.md or in --help.

What are all the pragmas? The only one I'm aware of is cabal-fmt: expand DIR [EXCLUDE...] where each EXCLUDE is formatted as -Module e.g. cabal-fmt expand: app -Main.

However, looking through the source code seems to indicate that there are more pragmas:

cabal-fmt/src/CabalFmt/Pragma.hs

Lines 58 to 64 in a7ef55e

	"expand" -> expandModules
	"indent" -> indent
	"glob-files" -> globFiles
	"tabular" -> return $ GlobalPragma $ PragmaOptTabular True
	"no-tabular" -> return $ GlobalPragma $ PragmaOptTabular False
	"fragment" -> fragment
	_ -> fail $ "Unknown pragma " ++ t

GitHub search seems to show more pragmas as well (at least a cabal-fmt: fragment and cabal-fmt: glob-files): https://github.com/search?q=%22cabal-fmt%3A+%22+extension%3Acabal&type=Code