Serialize `!important` in declarations.
preserve_{comments,whitespace} in parser, default on

Add the preser_comments and preserve_whitespace parameters to all
component value/rule/declaration/stylesheet parser functions, and change
the default value to True.
Stop normalizing WhitespaceToken.value, rename preserve* to skip*

… and revert the logic of preserve/skip.
Update docstrings.
More docs.
Changelog for v0.5