Turn --hide-error-context on by default

[ddfisher]

This diff provides a flag called --show-error-context in place of --hide-error-context and flips the default.

This also removes all context lines from tests so they do not need to be special cased. Here's why:

• It's much easier to write tests when not having to worry about error context, so this is nicer going forward.
• It avoids adding any more special casing to tests.
• This won't results in any git blame problems because the transformation is purely subtractive.
• This shouldn't be particularly difficult to review, because the transformations are split into separate commits.

[JukkaL]

This looks good to me. Just a few things:

• Let's also give @gvanrossum (and others) a chance to comment for a while.
• Add test for specifying the equivalent of --show-error-context in the config file (assuming it's supported; if not, we should add this option).
• Maybe add a short command line option for enabling error context, since it can sometimes be useful and writing --show-error-context is kind of inconvenient.

[gvanrossum]

I recommend keeping the --hide-error-context flag, for backward compatibility reasons, but with help=argparse.SUPPRESS. This allows enabling it from the command line when disabled from mypy.ini, and avoids breaking scripts that pass the old flag. Otherwise LG. (I don't feel the need for a short option.)

[JukkaL]

OK, we can leave out the short option. It will be easy enough to add in the future in case it feels like it would be useful to have.

[gvanrossum]

I'm merging this now while it has no merge conflicts yet. I'll open a new issue for keeping the old option.