[elm] Add support for Elm 0.19

Created by: eriktim

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Add support for Elm 0.19. An option elmVersion is added to set the desired Elm version (defaults to 0.19). @wing328 can you handle the CI stuff via #878 (closed)? I already added the Elm 0.19 binary to Shippable.

After this PR gets merged I will work on some other open Elm issues.