Implement an NPM-based environment builder

[ctrueden]

Once we have a JavaScript implementation of Appose (#3), we will naturally want to be able to build Appose environments that include packages installed via NPM, so that JavaScript tasks can be executed from child proceses, rather than only allowing to embed Java and/or Python in Node.js applications.

To build a node environment, we need to:

1. download and cache an npm installation (would rather not require it to be installed manually)
2. use Appose's copy of npm to build the environment, including the appose node package

See also #4 (Java/Groovy environment builder) and #5 (Conda/Python environment builder).