Add file watcher for development with live reload

[goncalotomas]

• Make mix grf.server build when starting
• Watch for changes in the input directory and call mix grf.build every time a file is modified.
• Watch for changes in the layouts directory and call mix grf.build every time a file is modified.
• Use plug_live_reload to reload pages automatically when changes are detected in the output files.
• Rewrite HTTP Server to use Plug.Router but not Plug.Static so that Live Reload works
• Fix or suppress lint issues (we are now using credo, had forgotten to add it)
• Remove all Application related code, spawn required processes from within grf.server
• Allow changing HTTP port via -p, --port CLI option

This makes it so that when a user changes a file, it will first be picked up by the file watcher, rebuilt into a new version of the file in the output directory which is then picked up by plug_live_reload.

There was an issue with the old way grf.server was being started that was not allowing live reload to happen which I still need to dig into a bit more.

[goncalotomas]

Things are starting to take shape but there are still a couple of things to do:

• Make Watcher watch for changes in layouts directory aside from input directory
• Rewrite HTTP Server to use Plug.Router but not Plug.Static so that Live Reload works
• Fix or suppress lint issues (we are now using credo, had forgotten to add it)

[goncalotomas]

@lcmen perhaps not too late, but we finally have a server that watches file changes and it even live reloads! 😄

[lcmen]

@goncalotomas thanks for the heads-up! Unfortunatelly, I've settled on Jekyll for now.