* Enable easy rebuild of native modules for unreleased electron

Sometimes when developing locally (as I found this morning) you want to rebuild native modules
against an unreleased version of Electron (in this case local master).  This PR adds a simple
script to launch a local HTTP server that generates and hosts the required header and checksum
files to rebuild modules.  So far only tested on macOS but should work on all OS's

* Allow custom port

* clean up serve script

* make optionality more obvious