Add support for elm 0.19

[daegren]

Elm 0.19 has been released and makes some significant changes to the build tools.

elm-make is now elm make, support for this should be added: Either through being able to set the executable name or by detecting the version of Elm being used.

See the upgrade docs

There is already a PR (#142) for this, just adding a ticket to make sure it's dealt with :)