Accommodate cookiecutter templates repository

[eudesbarbosa]

Background
We are planning to move ISA Tab cookiecutter templates to their own repository. Hence, we need to adjust the code to accommodate these changes.

Possible Solution
Add a submodule for the new repository - this way the templates are still a part of cubi-tk code. Disadvantage of the approach is that using --recurse-submodules is error-prone.
This change affects how we clone and update already cloned versions of the repo: i) the documentation should include git submodule instructions; and ii) it should reference a specific SHA in the target repo.

Alternative solutions

1. Let cubi-tk clone the template repository and then call cookiecutter() on the subdirectories. It wouldn't require submodule, but future releases could suffer as we would be tracking another repository version in the source code.
2. Create one repository per template and call cookiecutter() on the upstream template URLs directly - i.e., how it was "meant" to be used. The downside would be to track and maintain the 5+ loosely connected repositories.

Additional context
Related issue: #96