Support users defining/registering their own format directives

[jwodder]

(Including %{*}t subdirectives)

• Provide functions like register_plain_directive(directive, name, pattern, converter)
• Add a DirectiveRegistry class with methods for registering new directives so that we aren't modifying the global state?
  • Make the LogParser constructor "private" and have instances instead be constructed through a method of DirectiveRegistry, with a global function version provided that uses a private registry of default values only?
    • Alternatively, give the LogParser constructor a registry=None argument that defaults to the private default registry?
  • Give the DirectiveRegistry class parse() and parse_lines() methods, with the global functions operating on a private registry of default values only