Better parsing of SHELL

[dsherret]

Shell has the following structure:

SHELL ["executable", "parameters"]

Example:

SHELL ["/bin/sh", "-c"]
SHELL ["cmd", "/S", "/C"]

Instead of a MiscInstruction where this gets parsed as a BreakableString, it would probably be best to have this parsed as a ShellInstruction and have an elements: StringArray property (StringArray is from #17).

I can contribute this once #17 lands if this sounds good.

https://docs.docker.com/engine/reference/builder/#shell