Normalize input and output file names

[ryanelian]

• Right now the JS entry file name is correct: index.ts
• Change input site.scss name to index.scss
• Change default ouput site.css name to ipack.css
• Change default output bundle.js name to ipack.js

While technically a breaking change (version 5), the remedy should be easy enough for the tool users.

When: October release will drop Node 6 support in favor of Node 8 LTS, which should bump the major version. Use this moment to perform this change as well.

---

Allow renaming output css and js file names.

package.json:

{
  "instapack": {
    "jsOut": "bundle.js",
    "cssOut": "site.css",
  }
}

The above example is how you'd emit output files identical to instapack 4.

instapack should intelligently parse jsOut and cssOut: the fields can be set with or without .js or .css extension!

Also, fix concat output configuration to detect whether .js extension is given or not.