Changed APIs and some cleanups:

* New APIs anyconfig.api.single_load to load single config file and
  anyconfig.api.multi_load to load multiple config files
* new API anyconfig.api.load to load config file (path) or files (path
  pattern, e.g. /a/b/c/*.json)
* load_metaconf: Removed becauses its intended behavior and design is
  vague still.
* Some cleanups in anyconfig.api.dump[s]
followed API changes in anyconfig.api