Allow extensions other than .js for entry point

Created by: detrohutt

This is my proposed solution to #3052 (closed) and #3941 (closed).

I changed the checkRequiredFiles function to allow passing files with no extension. When a file with no extension is received, it's assumed to be a js-like file. All of the js-like file extensions will be checked.

I can add tests and such over the next few days but I wanted to get some eyes on this to tell me if I'm heading in the right direction.

Edit: If the number of potential disk reads is a concern I could also use fs.readdirSync() upfront and search the returned array for the different extensions.