add DisplayFormatter.active_types

allows kernel-side default whitelist of display formats.
set default active_types to plaintext only in terminal IPython
add missing warnings import
propagate deprecation of Formatter.plain_text_only
implement active_types via formatter.enabled attributes

it would be ideal if this could just be a property,
but I don't know how to make a trait a property.
simplify include/exclude conditions
use items, not keys, duh
expand active_types help string
Merge pull request #2981 from minrk/format_types

IPEP 10: kernel side filtering of display formats