Having a common base class allows users to catch all xylem
related exceptions in one handler.

 - extensive tests for config types
 - tests for other parts of config utils are still TODO

- new module `config` describes the global configuration

- `config` includes the logic to compute soruces and cache folders given command
  line arguments and options

- `arguments` module features help action, which uses pydoc.pager to display the
  help in a `less`-like scrollable manner on pty's

- help output includes new section `config arguments`, describing the arguments
  that correspond to a config file entry

- commands have a new hook to add to the config description; some logic common
  to all commands was moved to `commands.main`

- context objects like `InstallerContext` and `SourcesContext` now take a config
  dictionary as argument, which defaults to the global config sigleton if not
  passed explicitly

- high-level API used by the commands (`update`, `resolve`, `lookup`) is also
  parameterized by an optional config dict argument, which defaults to the
  global config