0.7.0: Templated manifests

Added support of templated manifests. Manifests can be written in
Jinja2 format to have an ablity specify some build time variables
such as:
- version
- name
- images
- manifests

For instance, the original templated manifest:
```yaml
name: "demo"
description: "Demo element"
schema_version: 1
version: "{{ version }}"
...
```

The manifest after build
```yaml
name: "demo"
description: "Demo element"
schema_version: 1
version: "0.0.1"
...
```

The manifest is considired as a template if its extension finishes at
`.j2` or `.jinja2`.