New open/close plugins; improvements

New

• autoclose_tree: new plugin automatically closes all of an account's descendants when
  an account is closed. [Red S]
• opengroup (experimental): new plugin to automatically open sets of accounts. [Red S]

Improvements

• rename_accounts: accept regexes for renames (#20) [Ankur Dave]
• rename_accounts: Support renaming within all directives, not just Transaction (#17) [Ankur Dave]
• rename_accounts: Copy only renamed directives for performance (#18) [Ankur Dave]
• long_short: use tolerance to determine whether to scale. [Red S]

Fixes

• #19 error when used in conjunction with beancount.plugins.auto_accounts. [Red S]