configuring traitlets. Highlights:
- Decorators are used, rather than magic method names, for registering
trait-related methods. See `using_traitlets` and `migration` for more
info.
- Deprecate `Trait(config=True)` in favor of
`Trait().tag(config=True)`. In general, metadata is added via `tag`
instead of the constructor.
Other changes:
- Trait attributes initialized with `read_only=True` can only be set
with the `set_trait` method. Attempts to directly modify a read-only
trait attribute raises a `TraitError`.
- The directional link now takes an optional `transform`
attribute allowing the modification of the value.
- Various fixes and improvements to config-file generation (fixed
ordering, Undefined showing up, etc.)
- Warn on unrecognized traits that aren't configurable, to avoid
silently ignoring mistyped config.