Add track_file_count decorator
Convert src to property

src is now a property because the map cache must be reset on every src
change.
Add MergeMapper
Handling empty maps and other bugfixes

Now there is a defined, documented behavior for empty maps that can be
influenced by the user.

Corrected task dictionary generation: "action" now is "actions" and a
list type.
GlobMapper asterisk must match at least one character.
Explained GlobMapper replacement behavior.