Templates for construi.yml

[lstephen]

Allow the fetching of templates for construi.yml.

For example:

templates:
  - https://github.com/lstephen/construi-templates/java.yml

This templates would allow the setup of common/standard build types for certain types of build (currently the work around is to do this within docker, e.g., lstephen/docker-docker-push).

The templates can be merged using a dict merge before construi.yml is processed.