Package-scoped variables, new instruction api.pckgscope(), consequent update of the coffeescript package

[TC827]

At this moment it is not possible to write a meteor package in coffeescript and strict mode. See this stackoverflow question.

This pull request contains the following changes:

• new instruction api.pckgscope() to be used in package.js to declare variables with package
  scope. This instruction is similar to api.export() with the difference that in this context there is no
  export.
• the consequent updates to the isobuild
• the consequent updates to the coffeescript package. The entire problem with share is now
  obsolete
• update to the documentation of the coffeescript package.
• small update of the packaging documentation

[apollo-cla]

@TC827: Before we can merge your pull request, you'll need to sign the Meteor Contributor Agreement: https://contribute.meteor.com/

[TC827]

Test cases: github:TC827/meteor_pr4907

[avital]

I think there's a much simpler fix to your issue by modifying packages/coffeescript/plugin/compile-coffeescript.js to place the declaration of __coffeescriptShare before "use strict" (which is already searched for).

In the longer term, we're planning to move towards ES6 modules for variable scoping instead of our in-house "linker" solution.

[stubailo]

Yeah, I don't think we are going to add this API to package.js, especially since Avi's fix above should work. I'd look at a PR with that strategy.