Allow autocompletion of fields

[dgasmith]

Change Summary

Modifies the __dir__ method to show fields as well as canonical functions and attributes. As the __dir__ method is explicitly for interactive use (see the docs) this should not effect other areas of pydantic and as such is implemented in a very straightforward manner. It is also possible to hide metafields and the Config object as well if desired.

Use case: Our users will often handle pydantic objects in Jupyter notebooks and interactive interpreters where this feature is quite useful when exploring objects that they are not familiar with.

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[codecov]

Codecov Report

Merging #398 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #398   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     14           
  Lines        2064   2068    +4     
  Branches      420    420           
=====================================
+ Hits         2064   2068    +4

[samuelcolvin]

awesome, thank you.