[Feature] Defines internal placeholders for current item / procedures to be consumed from plugins.

[stefanomondino]

As of today, Plugins defines custom implementations to be run before or after each item/procedure resolution.

This gives Murray developers a great control over how they implement each plugin, but lacks of control for Murray users when they want to use plugin details with some kind of context over current element being processed.

Lets take the Shell plugin used in an "after item replacement" phase. it's easy to add a command like echo Hello after each replacement, but what if a user wants to echo current Item destination path or something similar?

Murray Plugins should inject in provided context some special variable like _inputPath or _outputPath with current input properties, so that the plugin section of a BoneItem can contain something like echo {{_inputPath}}, with complete control over shell commands configurations.

• custom names should all start with a _, so that it's harder to collide with provided context
• in any case a custom name should overwrite a previously existing property inside provided context
• context override should shared between every plugin (future-proof), maybe in the Plugin base-class.

List of possible values:

• _name: name of current item / procedure being processed
• _sourcePath: complete path of source template
• _destinationPath: complete path of destination template