Add code coverage

[martinfrances107]

I am measuring my progress in porting this module into rust by the amount to code coverage I see...

That is compared to this module.

It is a simple change to the package.json file have a new test script, So that when I run

npm run coverage

I see this appended to the usual test result.

-------------------|---------|----------|---------|---------|------------------------
File               | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s      
-------------------|---------|----------|---------|---------|------------------------
All files          |   93.85 |    72.02 |   98.75 |   96.39 |                        
 d3-geo-voronoi.js |   93.85 |    72.02 |   98.75 |   96.39 | 31,300-315,474,489,495 
-------------------|---------|----------|---------|---------|------------------------

Its adds a new dependancy 'nyc' but it is a popular module with over a million weekly download
https://www.npmjs.com/package/nyc

I am about to generate a PR.