Enable specifying additional directives in library definitions

An option could be for libraries to specify if they should be a conditional include, so that their sources are only added when the library is present on the page instead of the current behaviour of always being added on every request. This would be one method to resolve the issue where a module may have optional libraries that are only included when a certain feature is enabled (e.g. IIRC webform has some libraries with external domains that are not used by default).