Fix launcher configuration

[mliarakos]

Fixes #47

Overview:

• Add empty default constructor to AppLauncher to prevent the Cannot create instance of class error
• Prefer loaded config over default config to correctly read the launcher configuration option
• Update launcher selection logic to be more in line with scala-maven-plugin:
  • use the launcher specified by the launcher configuration option
  • if that doesn't exist or is not defined, fall back to the first launcher from the list of launchers in the configuration
    • for scala-maven-plugin the first launcher is used only if launcher is not defined (vs. also if it doesn't exist)
    • I added the fall back logic with a new warning
    • I can change the behavior to exactly mirror scala-maven-plugin if needed
• Refactor out emptyLauncher in favor of an Option[AppLauncher]
• Add test case for launcher configuration

[tgodzik]

I completely missed this PR 😨 Thanks!

LGTM