Support moving the ~/.node-gyp/ directory

[nogweii]

I'm on a mission to clean up my home directory. Or, at least, the top level. Part of it is minimizing the number of hidden dotfiles.

It would be appreciated if node-gyp supported using an environment variable to override the path to the devdir, complementing the current --devdir argument.

[bnoordhuis]

It already does (or should): npm_config_devdir - that's how node-gyp normally inherits config settings from npm.

[nogweii]

Ah! I did not grok the source code well enough to catch that. It would be nice if it was called out more explicitly.

[bnoordhuis]

I wouldn't object to a pull request that adds a blurb to the README.