Enable custom error formatting

[dmtucker]

This adds a new plugin attribute (file_error_formatter) that defines a callable which is invoked when mypy finds errors in a file.
Here is an example (similar to the new test) of how it can be used to resolve #90 (cc: @markusschmaus, @jduprey):

import py


def custom_file_error_formatter(item, results, errors):
    """Include the file path before each reported error."""
    return '\n'.join(
        '{path}:{error}'.format(
            path=py.path.local('.').bestrelpath(item.fspath),
            error=error,
        )   
        for error in errors
    )


def pytest_configure(config):
    plugin = config.pluginmanager.getplugin('mypy')
    plugin.file_error_formatter = custom_file_error_formatter

[jduprey]

Thank you! I look forward to trying it out.