Parse extension names in .cabal files using Text rather than Read

This means we can now parse unknown extensions without falling over.
Indeed if the compiler supports the unknown extension then we can
even successfully use it. This gives us some forwards compatability
for new extensions.

Distribution/ParseUtils.hs

@@ -563,7 +563,7 @@ parseLicenseQ = parseQuoted parse <++ parse
 -- Hence the trick above to make 'lic' polymorphic.
 
 parseExtensionQ :: ReadP r Extension
-parseExtensionQ = parseQuoted parseReadS <++ parseReadS
+parseExtensionQ = parseQuoted parse <++ parse
 
 parseTokenQ :: ReadP r String
 parseTokenQ = parseReadS <++ munch1 (\x -> not (isSpace x) && x /= ',')