tests: test another regex in incoming (luacheck-style linter output)

incoming: make initial state = None

incoming: use named groups in start-of-the-string regex

linters: jshint config

checker: configurable allowed exit codes for linters

linters: luacheck config

linters: yamllint config

linters: flake8 config

checker: don't hardcode linter config, load it from files

checker: go back to using hg files with -I and -X Because -X is a bit simpler than a fileset that also excludes some files (to code and just visually). Also rename property to "include" for clarity.