Add support for node version manager

[pricejn2]

This is a feature request to enhance the npm and node component of the CSS feature added with BOA 3.1.2. Some incompatibilities between npm packages and certain versions of node, deem it necessary to provide node version switching capabilities.

NVM https://github.com/creationix/nvm/blob/master/README.md probably makes the most sense to accomplish this, as is parallels the current RVM approach used for Ruby.