New features:
- new tool.create_complete_files function that creates the completion
  files for bash and zsh.
- the Section class is now public, ConfigurationManager takes them as
  kwargs.
- new Section.reset_ with a similar role as ConfigurationManager.reset_

API changes:
- ConfigurationManager.subs_ is replaced by sections_
- ConfigurationManager.def_ has been removed, use the Section method.
- only sections can be set during the initialization of
  ConfigurationManager instances. Configuration files must be set with
  the set_config_files_ command.
- new ConfigurationManager.from_dict_ allows factoring a
  ConfigurationManager with a dict of dict of Section, like with the
  ancient API.
- ConfOpt now lives in the manager module
- subcommands as well as methods related to CLI are now handled by a
  new class CLIManager. Note that a parser is built upon initialization
  of an instance of this class. You no longer need to call
  build_parser_ explicitly (the ParserNotBuiltError has been removed).
- CLIManager.common and .bare replace the None and '' special
  subcommands.
- Subcmd now lives in cli and the initialization syntax is clearer.
- Subcmd.extra_parsers is replaced by sections
- an internal module contains low-level or very specific definitions
  that shouldn't be used by the end-user.

Bug fixes:
- Section.update_ will no longer attempt to update unexisting options.
- ZSH completion script: the +group feature of the _arguments function
  was only introduced in zsh 5.4. It is only used if loam can be sure
  that zsh >= 5.4 is installed.
- ZSH completion script: the = after an option name is no longer
  required.
- Section and ConfigurationManager inits ensure options and sections
  names are valid identifier and different from loam_sub_name.