added model docstring command

[josephmancuso]

This PR adds a new command:

$ craft model:docstring table_name

for me running

$ craft model:docstring users

generated this:

"""Model Definition (generated with love by Masonite)

id: integer default: None
name: string(255) default: None
email: string(255) default: None
password: string(255) default: None
remember_token: string(255) default: None
created_at: datetime(6) default: CURRENT_TIMESTAMP(6)
updated_at: datetime(6) default: CURRENT_TIMESTAMP(6)
customer_id: string(255) default: None
plan_id: string(255) default: None
is_active: integer default: None
verified_at: datetime default: None
"""

Closes #359

[coveralls]

Coverage increased (+0.004%) to 91.497% when pulling c65c48d on add-model-docstring-command into 652b79e on master.