* add travis-ci support to test py2 & py3...

* What about if the per-section inheritance references settings in the
  DEFAULT section of the inherited file?... perhaps a flag to indicate
  how to handle this?...

* There should be a command-line tool that converts iniherit-enabled
  INI files to flat "standard" INI files.

* Implement `write`... this could be done by tracking which config
  items are "dirty", and only saving those to the base INI...

* Allow ENV and SUPER default values to have special characters, e.g.:

    %(ENV:VARNAME:-%(defval)s)s

  or

    %(ENV:VARNAME:-%(defval\)s)s

* Make "%inherit" expansion support *recursive* iniherit-based expansion...
  eg it currently supports::

    value    = foo
    %inherit = configs/%(value)s.ini configs/%(ENV:VALUE)s.ini

  but not::

    value    = foo
    envval   = %(ENV:VALUE)s
    %inherit = configs/%(value)s.ini configs/%(envval)s.ini

* Allow quoting directly in %inherit during expansion, eg so that this::

    %inherit  = file.ini %(list1)s "%(list2)s"
    list1     = file2.ini file3.ini
    list2     = other file with spaces.ini

  loads the files:

    - "file.ini"
    - "file2.ini"
    - "file3.ini"
    - "other file with spaces.ini"

  instead of:

    - "file.ini"
    - "file2.ini"
    - "file3.ini"
    - "other"
    - "file"
    - "with"
    - "spaces.ini"
