ArrayBuffer and outputType support for Packer

[joshkel]

patchDocument's outputType parameter is very convenient: it allows me to use ArrayBuffer (for easier use from a Node.js worker), and it lets me easily change between output types via parameter if I ever need that. However, Packer doesn't support ArrayBuffer and doesn't offer an outputType parameter.

Suggestion:

• Add a Packer.pack or Packer.to method that takes an outputType parameter.
• Change the existing Packer.to... methods to use this new method.
• Optionally, add a Packer.toArrayBuffer method for consistency with the existing Packer.to... methods.

If this seems worthwhile, I can submit a PR.

[dolanmiu]

I like the idea

Let's go with option 3 and make Packer.toArrayBuffer so we don't create a major bump of docx. Can refactor later if need be in future in the next major

Sure yes, if you can submit a PR, that would be great!