Add Plug.Head

This Plug converts "HEAD" requests to "GET"
Merge pull request #9 from guilleiguaran/headplug

Add Plug.Head
Update to Elixir v0.12.3
Add a plug builder
Outline the new plug specification
Add missing plug wrapper
Convert all plugs to the new specification
Move to new sigils syntax
Move router to builder style
Optimize http method dispatching
Fix test with no body should not set content-type
Merge pull request #13 from ericmj/fix-test-nobody

Fix test with no body should not set content-type
Allow quoted expressions in plug options
add init to MyPlug example
Merge pull request #15 from cromwellryan/add_init_to_myplug

add init to MyPlug example
Release v0.3.0