Support for multiple templates for the same project

[KostyaEsmukov]

The idea is to allow templates be composed. For example, one project could be synchronised with the following templates:

• Base Python project (which includes Python packaging, linting, testing)
• A Django/Flask/Aiohttp application (excluding the base Python project stuff)
• Travis CI/GitLab CI/Jenkins

This is quite cumbersome to achieve with plain cookiecutter, as after applying the first template there already are files in the target project, so applying another template on top of that wouldn't work. But with scaraplate it's easier, given that it can rollup templates on top of already existing projects.

In fact, this can already be achieved right now, the only thing to consider is separating cookiecutter context types. But currently it is impossible to write multiple template contexts to the same file (e.g. .scaraplate.conf) using the built-in CookieCutterContext implementations.

DoD:

• ScaraplateConf and SetupCfg support multiple templates by separating the contexts
• the docs are updated to reflect the fact that scaraplate templates can be composed.