Add configurable NODE_PATH support and improve module resolution

[charlespwd]

Proposed changes:

• Use the node.js module resolution algorithm
• Add support for $NODE_PATH
• Add support for configurable g:vim_node#node_path variable
• Add .editorconfig to prevent mixed indentation

So if you ever defined src as your NODE_PATH for a project, you'll now be able to resolve your fancy imports with gf.

fixes #11

[moll]

Hey, thanks, but I'll reopen this as a reminder to check it out someday. :)