tweak formatter.for_type

- for_type(cls) nondestructively queries the current formatter (previously disallowed)
- for_type(cls, None) clears the current formatter (previously queried current)
exercise formatter.for_type in tests
add lookup logic based on apptools.type_registry

adds

- formatter.lookup(instance)
- formatter.lookup_by_type(type_or_type_string)
- formatter.pop(type_or_type_string)

changes:

- support 'module.name' in formatter.for_type
more formatter type lookup tests
remove use of private type_printers API
add formatter changes to what's new
another pass on formatter lookup

- pop takes a default value (like dict)
- support `type in Formatter`
- allow lookup by type string (for consistency)
update formatter what's new
formatter docstring consistency
Bug fix for #4643: Regex object needs to be reset between calls in tooltip.js
Add HTTP handlers for nbconvert
Add tests for nbconvert HTTP service
Add menu entries for getting converted views of a notebook
Add MIME types to nbconvert exporters
Add option to download as reST
Condense raw_mimetype and mime_type traitlets into output_mimetype
Capture server logs when running notebook tests
Make output_mimetype accessible from the class.

Traitlets are only accessible by instantiating the class. There's no
clear reason that this needs to be a traitlet, anyway.
Add GET /nbconvert to list available formats
Describe nbconvert in notebook UI for whatsnew