Feature request: Test Plan templates

[atodorov]

Be able to create a Test Plan document with a collection of test cases inside of it. Then when cloning (or creating new one and choosing the existing one as template) the system will perform variable substitution (see #227) and clone all test cases for me.

In this way we can define some standard set of test plans to get you started and you can clone & extend them in the future. (e.g. testing a mobile app).