* add documentation about:
  * Manager.default
  * Email.structure
  * Modifiers
  * ... and much more

* add `less` support for CSS via lesscpy

* add ability to add modifiers on a per-email basis
  ==> maybe controlled/configured by <email:modifier .../> ?...
  ==> or possibly by email.spec ?...

* add a MIME prolog? eg:

    This is a multi-part message in MIME format.

  *just in case* some email is received by a non-MIME aware reader.
  (say whaaa?)

* switch to use asset.isstr() -- search for isinstance()...

* the PgpModifier implements PGP/MIME (RFC 3156), add support for:
  * inline-PGP

* add S/MIME (RFC 5750) modifier for certificate-based encryption
  (makes much more sense in server environments)

* technically, this package is not zip-safe, but *only* for testing
  of the pgp modifier (since it uses gpg, an external program, which
  needs access to the test_data/gpg-* folders). what to do about that?

* add a Modifier base class parameter 'failaction' = {'abort', 'ignore'}
