All Readers - Allow or Forbid Fetching of External Images

[oleibman]

Add to all readers the option to allow or forbid fetching external images. This is unconditionally allowed now. The default will be set to "allow", so no code changes are necessary. However, we are giving consideration to changing the default.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary