Add standard settings

[ydah]

The following settings have been added.

• Set parallel execution to true
• Change format to progress

[mike-burns]

Documentation

• parallel passes down to Rubocop, and it does this:

  Use available CPUs to execute inspection in parallel.

• progress also passes down to Rubocop, and prints a . for each clean file.

Both of these are the Rubocop defaults. Standard overrides the defaults:

• parallel interacts poorly with stdin mode. As such, it is forcibly overridden in stdin mode, and the default is not to use it.
• progress doesn't mention standard, which seems to be why the Standard::Formatter was introduced.

Analysis

I don't even use Standard, but it runs as part of CI. Sure, I'll merge these changes, and if someone complains I'll back them out.