[config] add configuration system to define application settings

[palazzem]

Overview

Closes #17

Adds a conf module to define application settings. Each setting is defined with an Option class that includes built-in validators and is extendable to add more. Option initialization exposes the following kwargs:

• default: to define the default value
• allow_null: if None value is allowed or not
• validators: a list of functions that are executed to validate option's value

Example

from elmo.conf.config import BaseConfig

class Settings(BaseConfig):
    url = Option(default="http://example.com", allow_null=False)
    home = Option()
    dry_run = Option(validators=[is_boolean])

settings = Settings()
settings.is_valid()  # executes all options validators

[palazzem]

This PR includes application settings via elmo.settings.Settings class. Application configs are available in the following module:

from elmo.conf import settings

settings.base_url = "https://example.com"