Wip/1026 validate task

[muuki88]

This is an initial draft to provide helpful feedback to users
during their configuration phase with sbt-native-packager.

The intention is to give a short explanation why a warning or
error is triggerd and a detailed description how to fix the issue.

Example 1 - no archetype

A common issue is that users don't enable an archetype, because they directly skip to the format plugin documentation and simply enable DockerPlugin or DebianPlugin.

A basic mappings validator can catch this issue and print a helpful error message.
Image a user a

• enablePlugins(UniversalPlugin)
• $ sbt universal:packageBin

Gives the output

[error] [1] You have no mappings defined! This will result in an empty package
[error] Try enabling an archetype, e.g. `enablePlugins(JavaAppPackaging)`