Fixes #3615

• a lot of refactoring (actually most of them are just moved/extracted).
• remove PRETTIER_LOG_LEVEL environment variable, the logger is not global now.
• add a showInternal option for supportInfo, which is a filter for cliSomething used in CLI.
  • centralize infos
• support usage prettier --plugin foo-plugin --help.
• support detailed usage prettier --plugin foo-plugin --help foo-option.
• support external options from CLI prettier --plugin foo-plugin --foo-option.
• support external options from config file.
  • .prettierrc: { "plugins": ["foo-plugin"], "fooOption": "bar" }
• does not support external plugins from config file but want to pass its options to CLI, vice versa.
  • .prettierrc: { "plugins": ["foo-plugin"] } + CLI: prettier --foo-option