Add support for spot colors

[aslakhellesoy]

What kind of change does this PR introduce?

This adds support for spot colors - see PDF Association and Adobe for details.

A new PDFDocument.addSpotColor method has been added:

const doc = new PDFDocument();
doc.addSpotColor('PANTONE185C', 0, 100, 78, 9)

// Draw a triangle with the spot color
doc
  .moveTo(100, 150)
  .lineTo(100, 250)
  .lineTo(200, 250)
  .fill('PANTONE185C');

The code was originally written by @ziaenezhad who mentioned it in #756
Then @sekforde submitted #1457 which was closed unmerged.

My code is based on the two contributions above, but I've also added a test, and updated the documentation and changelog. So hopefully this in a more mergable condition.

Checklist:

• Unit Tests
• Documentation
• Update CHANGELOG.md
• Ready to be merged

[aslakhellesoy]

Not strictly related to this PR, but as I was running the tests, my new test initially failed, and I had to change this to make the test error correctly.

[ErnestoBorio]

@aslakhellesoy Hi! thanks for this fork! I'm trying to import it into my project but for some reason Node can't find it, any ideas?
My repo: https://github.com/ErnestoBorio/pdfkit-example
The errors I get in JS and TS:
main.ts:1:25 - error TS2307: Cannot find module '@openartmarket/pdfkit' or its corresponding type declarations.

Error [ERR_MODULE_NOT_FOUND]: Cannot find package '/Users/petruza/Source/pdf/node_modules/@openartmarket/pdfkit/' imported from ./main.js