Support lazy importing/loading (of formatters, etc.)
Improve handling for unknown formats (formatters).
Support cases w/o format option.
Use lazy imports for formatters (to speed up start-up time).
FORMATTERS:
  * NEW: steps.doc formatter, shows documentation of step definitions (related to: #55).
  * NEW: steps.usage formatter, shows where step definitions are used.
  * RENAMED: json-pretty, tag_count, tag_location => json.pretty, tags, tags.location
  * help: Shows now a better formatted list (improve readability).

location: Use now a FileLocation object (was: string), required for ordering.
load_step_definitions: Are now sorted before loading (due to: Linux, ...).