Add filter for overriding image placeholder

[cr0ybot]

Summary

This PR can be summarized in the following changelog entry:

• Added filter native_lazyload_placeholder to allow overriding the placeholder path.

Addresses issue #22

Relevant technical choices

Added a filter named native_lazyload_placeholder roughly following the precedent set by native_lazyload_fallback_script_enabled. This filters the lazyload element's src attribute and overrides the hardcoded PLACEHOLDER_PATH.

Also, please note that I assumed this would be a minor version bump, so the filter documentation uses @since 1.1.0.

Checklist:

• [x] My code is tested.
• [x] My code is backward-compatible with WordPress 4.7 and PHP 7.0.
• [x] My code follows the WordPress coding standards.
• [x] My code has proper inline documentation.