format: good things to fix before 0.2

[avh4]

A few various things that seem good to fix before the 0.2 release:

• retain whether or not a custom type's variants are exposed in exposing listings
• allow integer literals to retain hexadecimal format
• allow string literals to retain multiline (triple-quoted) string format
  • make sure it works correctly w/r to New behaviour for multiline strings #151 (though I'm not sure I understand the feature well enough to think of all possible edge cases to consider)
• retain comments before and between import statements