Fix filtering for files method

[alkamin]

Overview

This PR adds extension filtering to the files method within the CLI and then uses that functionality to properly filter for .js files when generating the common index for each package.

Checklist

• [NA] Relevant documentation pages have been created or updated
• Description of PR is in an appropriate section of the changelog and grouped with similar changes if possible

Testing Instructions

• Pull down the branch
• Delete the contents of the core package's index.common.js file
• blaster i
• Verify that the README.md file did not show up in the index.common.js file and that the other files were properly referenced

Closes #119

[designmatty]

+1