fix files behavior on node, and generally remove redundancy in the files key

[ashleygwilliams]

fixes #199 (FOR REAL THIS TIME)

this PR does several things:

• passes the value of the target flag to the write_package_json method
• optionally includes a _bg.js file in the files key if the target is nodejs
• updates files key to NOT include the file in main (it is unecessary)
• adds a test to ensure correct behavior on node
• cleans up tests to test for correct files key

[fitzgen]

👍

[fitzgen]

Nitpick: I think this would be nicer as

if let Some(ref f) = js_bg_file {
    files.push(f.to_string());
}