Changes:
- Move InvalidListRepliesPolicyError, InvalidTimelineCategory,
InvalidStatusVisibility and InvalidStatusContentTypeError type to the
model package.
- Clean up some code in regards to the parsing of the Enum types.
- Clean up the error messages sent back to the user.
- Use colons instead of semicolons when unwrapping error messages.
- Print errors to Standard Error (os.Stderr)
Allow users to specify their preferred configuration directory by using
the top level flag (a.k.a 'global flag') --config-dir. This also
introduces the design of baking top level flags into the executor.