Proposal for supporting call-overload errors

[ollij93]

Proposed fix for #44

Adds a new severity symbol "O" (i.e. call-Overload). This new severity acts as "E" but activates suppression of note messages on the same line. Documentation and testing updated to reflect this behaviour.

Hopefully this proposal is useful. If there's a better design to help handle this case I'd be happy to help out with any alternative implementation suggestions!