Add settings object

[berislavlopac]

Right now, the settings are supposed to be read from a source and kept in (a kind of) a dictionary. The typing is limited, and there is no enforcing types if the source keeps everything in string, like it's the case for environment variables or ini files.

The idea is to add the support for specifying a set of predefined settings, in a form similar to a dataclass or attr.s, something like:

@dataclass
class Config:
    some_value: int
    some_value_with_default: str = "foo bar"

config = configuration(Config)

... or something similar.

Basically it would "enrich" the class with various functionalities useful for configuration:

• Methods to ingest values from files and environment, similar to what we have now, but with full conversion to the right types.
• Option to allow or ban values not defined in the specification, and how to convert them.
• Automatic construction of argument parsers for CLI applications.