Match messages with multispans and multiple messages on a line

[pczarn]

Fixes #41

Example of an expected error that matches 2 actual errors

// Should get errors for both 'Some' and 'None'
use std::option::Option::{Some, None}; //~ ERROR*2 unused import

[pczarn]

I'm not sure about making the change in behavior for multiple messages. Should the following expected error match two actual errors?

// Should get errors for both 'Some' and 'None'
use std::option::Option::{Some, None}; //~ ERROR unused import