feat(iopipe handler files): output multiple iopipe handler files

[coreylight]

By creating a new iopipe handler file per function, the plugin can support advanced use cases such
as webpack individual packaging.

fix #64

• Create new iopipe handler file per function
• Introduce del pkg for glob file removal
• Remove a couple of end-to-end tests from index.test.js as they are fully covered in the default project tests
• Add name mismatch handler/test to ensure that any handler/file naming paradigm is supported.