Improve importation support

Main changes

• Support non-flat directory structure, as long as the import syntax is:

from a.b.c import d

• New documentation section.
• import is now a root in the default taxonomy.
• Internals: rename user type ProgramName (and derivative's) into ProgramPath.
• The key name for the --format option of paroxython recommend is now path.
• A new key name is added.

In other news

• The success, warning and error messages are now colorised (on certain terminals).
• New CLI error messages when referring to a non-existent file or directory.