Always float comments to the top of field values

[tfausak]

Fixes #70.

Previously Gild would only float comments to the top of a field if Gild knew how to parse the field (like build-depends) and the field parsed successfully. Otherwise Gild would leave the comments where they were (although it would re-indent them). Now Gild will always pull all comments up to the top of a field, regardless of if Gild knows how to parse the field or if the parsing succeeded.

This is a change in behavior. Hopefully it's easier for users to understand how Gild behaves.

Before:

some-field:
  foo
  -- bar
  qux

After:

some-field:
  -- bar
  foo
  qux

In the future, it's possible that Gild will make an effort to keep comments where they were. However in general this is a difficult problem and in some cases it doesn't have a clear answer. So for now this predictable behavior is preferable.