Preserve "important" CSS comments

According to the issue https://github.com/sindresorhus/strip-css-comments/issues/4 by @terribleplan,  here are the changes to preserve the "important" CSS comments (beginning with the characters `/*!`) in the output and to allow the users to change this new behaviour by setting the option `preserve` to `false` or by using the option `--all` in CLI to strip all comments without any exceptions.