Add support for variables in the project descriptor

Having support for variables in the project descriptor would bring better user experience. For example, when creating a project from template, parts of the descriptor might refer to the project name, like post-build commands. Currently, the user would need to modify manually the parts of the descriptor that depend on project specific things.

Another interesting use would be to allow getting environment variables for post-build commands. For example, when secrets are required to access certain APIs.