Allow injection of modules that do not have corresponding files

Hey,

In using this module was hoping to inject files that would be required dynamically, however I seem to be getting a file error when doing so.

I wrote an addition to add an option `ignoreMissing` to essentially allow the failing call to "require-like" to fail silently and fall back to the current key as the filename. Leaving the option with a default that preserves the current functionality.

I took a shot at adding a test for the option, adding "test-ignore-missing.js".