This commit promisifies following APIs:
BaseViewer#importXML
BaseViewer#importDefinitions
BaseViewer#open
BaseViewer#saveXML
BaseViewer#saveSVG
Modeler#createDiagram
Related to https://github.com/bpmn-io/bpmn-js/issues/812
BREAKING CHANGES:
* Users are now expected to have Promises either by default or
polyfilled as the APIs return a Promise now.