Add ability to turn checks off by default
Prevent unintended behaviour modifying options.ignore

Previously, it was entirely plausible for us to remove something from
options.ignore overzealously. This is more confined logic and much more akin
to what I was intending the behaviour to be.
flush() for each print()
Fix missing import
Merge branch 'fix-17' into 'master'

flush() for each line

Fixes #17

See merge request !16
Merge branch 'feature/register-optional-checks' into 'master'

Allow for optional checks

See merge request !14
Add --output-file option

Closes #15