add generate mode and new //counterfeiter:generate directive

[joefitzgerald]

This adds a //counterfeiter:generate directive and a corresponding -generate flag that can be used to look for and execute the directives.

If you are using many //go:generate go run github.com/maxbrunsfeld/counterfeiter/v6 directives in a single package, go will execute counterfeiter multiple times and cause extraneous file operations searching through all lines in each file in the package. This cost is more impactful for packages with many files and for packages with large files (as measured by line count or length of lines).

If you wish to avoid these costs, you can now have a single go:generate directive in a package:

• //go:generate go run github.com/maxbrunsfeld/counterfeiter/v6 -generate

Then, replace instances of:

• //go:generate go run github.com/maxbrunsfeld/counterfeiter/v6 . YourInterface with:
• //counterfeiter:generate . YourInterface

The performance gain here is relatively small, 0 <= x <= 5% on packages I tested with.

Related: #120 and #123.