Disallow string concatenation when using __dirname and __filename (no-path-concat)

[feross]

See: http://eslint.org/docs/2.0.0/rules/no-path-concat

Replace this:

var fullPath = __dirname + "/foo.js";

With this:

var fullPath = path.join(__dirname, "foo.js");

From eslint docs:

However, there are a few problems with this. First, you can't be sure what type of system the script is running on. Node.js can be run on any computer, including Windows, which uses a different path separator. It's very easy, therefore, to create an invalid path using string concatenation and assuming Unix-style separators. There's also the possibility of having double separators, or otherwise ending up with an invalid path.

In order to avoid any confusion as to how to create the correct path, Node.js provides the path module. This module uses system-specific information to always return the correct value.

[feross]

# tests 427
# pass  395
# fail  32

7.5% of packages either don't work on Windows, or their tests don't work on Windows.

I think we should enable this, because even though this will make people change their code, this is actually a real problem – a programmer error — in their code that should be fixed.

[feross]

After fixing up all the packages that I have push access to, only 21/427 (5%) are failing. All the cases I fixed up were real cases where that code wouldn't work in a cross-platform way on Windows.

I think this is a good rule.

[jprichardson]

ACK. I think this change makes sense.

[feross]

Released in standard 6.0.0.