Migrate plugins to Pretzel 1.0 [WIP]

Essentially, we moved from System.ComponentModel.Composition to System.Composition v1.2.0. We are targeting net451 & netstandard2.0 for now and we plan to go net462 to reduce the effort, so you should multi-target as well or at least target netstandard2.0.

All extensions must have the explicit Export attribute and not just inherit from the interface. Example:

[Export(typeof(IAdditionalIngredient))]
[Export(typeof(IHaveCommandLineArgs))]
public class AzureHostSupport : IAdditionalIngredient, IHaveCommandLineArgs

ISiteEnginInfo and ICommandInfo doesn't exist anymore, you just have to use the corresponding attributes SitenEnginInfoAttribute and CommandInfoAttribute.

script plugin

ScriptCs isn't working anymore (but will be replaced).

command line plugins

See #347 for details.

Parameters.Path have been replaced by Parameters.Source.