Aliasing API for #describe

This adds the ability to configure aliases for #describe with default metadata, as requested in #493.

* Renames `ExampleGroup.describe` to `.example_group` (and aliases it back to `.describe`).
* Adds `Configuration#alias_example_group_to`, which accepts default metadata and a `:toplevel_alias` option.
* Adds `--toplevel-off` flag, which avoids extending the main object with the DSL methods (i.e. #describe and defined aliases).