This allows for users to configure fatal-warnings to be either a list of
warnings or simply True to trigger all configurable warnings as errors.

This commit deprecates the use of fail-on-overlap within project.conf,
this will now use the fatal-warnings configuration item.

plugin.py: get_warnings() is now available for plugins to override and
return their configurable warnings.

tests: This modifys the tests/frontend/overlaps.py tests to support the
new fatal-warnings configuration. Backwards compatibility is also
tested for `fail-on-overlap`

_versions.py: BST_FORMAT_VERSION bumped to 12 for fatal-warnings

Fixes: #526

This follows the implementation of configurable warnings.

This includes detailing the use of `fatal-warnings` within project.conf

This also adds a deprecation notice to the fail on overlaps section.

This adds multiple tests for custom plugin warnings and core warnings,
providing checks for both cases which should cause warnings and errors
when configured as fatal.