Download NodeJS if not present by default, or at least give better warning

[rnett]

Currently, if you try to use the plugin without node installed, the npm-install task fails. Unless you have downloadNodeJsVersion = "latest".

It would be nice to either make downloading the latest version the default behavior if node isn't found, or mention downloadNodeJsVersion more prominently in the docs, and especially in the warning message if npm isn't found.