🔧 Add versioned defaults

[nevans]

Requires #300.

Copied from the rdoc:

Versioned defaults

The effective default configuration for a specific x.y version of net-imap can be loaded with the config keyword argument to Net::IMAP.new. Requesting default configurations for previous versions enables extra backward compatibility with those versions:

  client = Net::IMAP.new(hostname, config: 0.3)
  client.config.sasl_ir                  # => false
  client.config.responses_without_block  # => :silence_deprecation_warning

  client = Net::IMAP.new(hostname, config: 0.4)
  client.config.sasl_ir                  # => true
  client.config.responses_without_block  # => :silence_deprecation_warning

  client = Net::IMAP.new(hostname, config: 0.5)
  client.config.sasl_ir                  # => true
  client.config.responses_without_block  # => :warn

  client = Net::IMAP.new(hostname, config: :future)
  client.config.sasl_ir                  # => true
  client.config.responses_without_block  # => :raise

The versioned default configs inherit certain specific config options from Config.global, for example #debug:

  client = Net::IMAP.new(hostname, config: 0.4)
  Net::IMAP.debug = false
  client.config.debug?  # => false

  Net::IMAP.debug = true
  client.config.debug?  # => true

Named defaults

In addition to x.y version numbers, the following aliases are supported:

:default
    An alias for :current.

    NOTE: This is not the same as Config.default. It inherits some attributes from Config.global, for example: #debug.
:current
    An alias for the current x.y version's defaults.
:next
    The planned config for the next x.y version.
:future
    The planned eventual config for some future x.y version.

For example, to raise exceptions for all current deprecations:

  client = Net::IMAP.new(hostname, config: :future)
  client.responses  # raises an ArgumentError