Support arbitrary launch process commands

[runesoerensen]

This PR changes how the launch.toml is parsed, to allow arbitrary process command lists (while retaining the current special handling for bash processes):

• The behavior for launch.toml command definitions like ["bash", "-c", "cd foobar/; ./foobar"] is unchanged. The third argument is used to define the process used by this buildpack (e.g. when detecting default process types in bin/release).
• Commands that do not rely on bash wrapping are now supported by the launch.toml parser. Every argument is checked for special characters, and double-quoted as needed.
• This prepares the buildpack to support these changes: Refactor test launch process definition buildpacks-dotnet#240.

Also see these related discussions:

• Refactor test launch process definition buildpacks-dotnet#240 (comment)
• Refactor test launch process definition buildpacks-dotnet#240 (comment)