[Feature] `ClickException` message without color would be helpful

Similar to how some runtime environments don't support colors, it may be helpful to programmers that the non-colorized rendering of the ClickException message was available... something I was thinking when encountering and working around #2193.

Color shouldn't matter, nor should creating subclasses be necessary for verifying that the correct user-facing ClickException was raised with a certain message, for example in a test:

"""Tests for ClickException and exception handling."""
import click

def test_exception_message_no_color():
    """Base Exception message should be plain text."""
    exception = click.ClickException(click.style('red', fg='red') + ' text')

    assert 'red text' in str(exception) # fails currently

I'd think that __str__, and the possibly the message property too, should be rendered as plain text. I do have a simple StringIO patch that works, as well as just creating a subclass for my code if click is unpatched.

Start at ClickException.str and its message property, using the issue's test_exception_message_no_color example as the expected behavior. Check how click.style and StringIO currently affect the exception text, then add coverage showing that the user-facing message is plain text without color codes.