Specify rescue handlers and error formatters using a hash

[robertopedroso]

As requested in #450, this PR adds the with syntax to rescue_from and error_formatter:

rescue_from ArgumentError, with: rescue_argument_error
error_formatter :custom, with: CustomFormatter

But retains backwards-compatibility:

rescue_from ArgumentError do |e|
  ...
end

error_formatter :custom, lambda {|...| ... }

[dblock]

Good. Do you think we should change the documentation to reflect the with: syntax? IMO this creates a better way to extend the DSL with additional options moving forward. Can you make that change if you agree?

[robertopedroso]

I agree that the documentation should be updated. I'll push commits for the YARD documentation in the morning. Should I also document this syntax in README.md?

[dblock]

Yes, please. README in general is much more important than the inline doc IMO, but I am sure many people feel otherwise.