new version v5.1.0 of Html2Pdf

• add support of css page-break-before:always and page-break-after:always on DIV tag, based on PR #190 and PR #204
• add no-html mode for debug output
• add php 7.2 compatibility
• add some unit tests, better coverage
• fix lots of bad method names and minor improvements, from PR #147
• fix bad cleaning after output or exception
• fix bad format detecting on page` tag - issue #260
• fix changelog file