Add show-memory-usage script

[mjethani]

The show-memory-usage.js script shows the memory usage of Node.js at various stages of using the publicsuffixlist.js package.

npm run show-memory-usage
npm run show-memory-usage -- --heap-snapshot

Use --heap-snapshot to get heap snapshots for the initial and final stages. Load the .heapsnapshot files in Chrome's developer tools for analysis.

Output:

Initial:
 *  Heap used: 2,372 KiB
 *  Heap total: 4,972 KiB

On import():
 *  Heap used: 2,647 KiB
 *  Heap total: 4,972 KiB

On parse():
 *  Heap used: 5,663 KiB
 *  Heap total: 9,104 KiB

On getDomain('example.com'):
 *  Heap used: 5,672 KiB
 *  Heap total: 9,104 KiB

On getDomain('example.s3-website.us-east-2.amazonaws.com'):
 *  Heap used: 5,678 KiB
 *  Heap total: 9,104 KiB

On getDomain('this.is.a.very.long.hostname.that.does.not.have.a.public.suffix'):
 *  Heap used: 5,682 KiB
 *  Heap total: 9,104 KiB

After GC:
 *  Heap used: 2,542 KiB
 *  Heap total: 7,580 KiB

[mjethani]

Note that the .npmignore file is independent of the .gitignore file and replaces the latter as far as npm is concerned.

This file must contain anything that is not to be published.