message: Added typescript defs (pptxgen.d.ts)
diff: - Bugfix/enhancement for EncodeXML in speaker notes text [\#373](https://github.com/gitbrent/PptxGenJS/pull/373) ([travispwingo](https://github.com/travispwingo))
  **Implemented Enhancements:**
  +- Typescript definitions have been created for the PptxGenJS API Methods (`pptxgen.d.ts`). ([gitbrent](https://github.com/gitbrent))
  - New Feature: Placeholder support in Master Slides [\#359](https://github.com/gitbrent/PptxGenJS/pull/359) ([conbow](https://github.com/conbow))
  - New Feature: Speaker Notes [\#239](https://github.com/gitbrent/PptxGenJS/issues/239) [\#361](https://github.com/gitbrent/PptxGenJS/pull/361) ([travispwingo](https://github.com/travispwingo))
  - New Chart Option: `displayBlanksAs` [\#365](https://github.com/gitbrent/PptxGenJS/pull/365) ([guipas](https://github.com/guipas))
  