Allow aliases for keys in usm.json and card.json

[frederikheld]

This visualization tool is designed for the use case "User Story Mapping" and therefore all keys follow the common naming in User Story Maps.

The visualization methods works great for other kinds of mapping as well though. For those kinds of mapping the naming might be different. So it would be great to use different names for the keys in usm.json and card.json.

Possible solution:

This might be done in a config file that defines the mapping between the original name and the alias.

Example:

activity=myaliasforactivity
step=myaliasforstep
story=myaliasforstory
story.description=details