Adding convenience function to get default settings

[domanchi]

Summary

One of the primary motivations of the v1 redesign is to enable developers to more easily extend and integrate this tool into their ecosystem. However, the new Settings object lends itself to a gotcha: by default, it starts off uninitialized and as such, it's easy to fall into the trap of scanning without any plugins enabled.

This PR attempts to make this easier. I've added instructions in the README, but now, developers can just do:

from detect_secrets import SecretsCollection
from detect_secrets.settings import default_settings

secrets = SecretsCollection()
with default_settings():
    results = secrets.scan_file(...)

This abstracts away the pain of figuring out which plugins they want to initialize themselves.

Testing

Automated.