Docs: add twitter and mailling list links
StringChecker: Fix failing test
StringChecker: Revert fix for failing test
Errors: Simplify rules debugging and prevent crashes in error reporters

- `add` method now throws on calls with invalid values
- set 0 as column in rules/disallow-mixed-spaces-and-tabs
- `add` throwing tests for add method

Fixes #743
Closes #744