Initial support for test/suite attributes
Print the message for skipped tests, if any (it just prints the first message it sees)
Add a string_attr type and clean some things up
Add support for list_attrs
Simplify compiling suites by moving more logic into compiled_suite
Allow filtering tests by attribute
Add tests for attributes
Move attribute filters to src/libmettle, since they don't make sense for header-only builds
Improve tests for attribute unite()
Allow merging of list_attrs instead of overriding
Be more efficient when building up the list of explicitly-shown attributes
Allow negating attribute filters
Remove unnecessary operator == for attr_list
Add tests for command-line parsing of attribute filters
Use a proper parser for parsing attribute filters
Don't emit started/ended_suite events if all the children are hidden
Add documentation for attributes
s/attr_list/attributes/g