Characterization: add default commands

One or more default normalization commands can be defined, by defining
FPRules with the purpose 'default_characterization'. As with
normalization, these will be called if no matching FPRules can be found.
This also allows for characterization rules to be applied to files that
couldn't be identified.

FITS will be the default characterization command in 1.1; others can
be introduced, or customized by users.
Characterization: add fiwalk plugins