Add Flags to control which sources are used

[zagaris]

Context:
This feature is listed as a to-do item in the project's README.

Problem:
Currently, the tool uses a predefined set of subdomain sources for enumeration, and all sources are always included. Users cannot enable or disable specific sources, making the tool less customizable for specific use cases. For example, users might want to exclude slower sources or ones that require API keys.

Solution:

• Implement a flag for each source to toggle its inclusion, as suggested in the README.
• All sources should be enabled by default unless explicitly disabled by the user.
• Update the README to reflect this functionality.