match: add optional warn argument Occasionally the matcher will want to print warning messages instead of throwing exceptions (like if it encounters a bad syntax parameter when parsing files). Let's add an optional warn argument that can provide this. The next patch will actually use this argument.

match: add source to kindpats list Future patches will be adding the ability to recursively include pattern files in a match rule expression. Part of that behavior will require tracking which file each pattern came from so we can report errors correctly. Let's add a 'source' arg to the kindpats list to track this. Initially it will only be populated by listfile rules.

check-code: reintroduce str.format() ban for 3.x porting In their infinite wisdom, the Python maintainers stripped bytes of its % and format() methods for 3.x. They've now added % back to 3.5, but format() is still missing. Since we don't have any particular need for it, we should keep avoiding it.