Support using other variables in variable files.

Fixes #28.

examples/run-example.sh

@@ -6,4 +6,9 @@ echo "Building..." >&2
 $(dirname $0)/../.buildkite/build.sh ${VERSION} 2>&1 1>> ./.build.log
 [[ $? != 0 ]] && echo "Build failed! (inspect '.build.log' for details)" >&2 && exit 1
 
-source $(dirname $0)/../deployster.sh --no-pull $@
+DVERSION="${VERSION}" source \
+    $(dirname $0)/../deployster.sh --no-pull \
+                                   --var bar="hi!" \
+                                   --var-file vars.1.yaml \
+                                   --var-file vars.2.yaml \
+                                   $@

examples/vars.1.yaml

@@ -0,0 +1 @@
+foo: bar-is-{{ bar }}

examples/vars.2.yaml

@@ -0,0 +1 @@
+chu: foo-is-{{ foo }}

src/deployster.py

@@ -38,10 +38,11 @@ def __init__(self) -> None:
 
     def add_file(self, path: str) -> None:
         with open(path, 'r') as stream:
-            if path.endswith('.json'):
-                source = json.loads(stream)
-            else:
-                source = yaml.load(stream)
+            environment = jinja2.Environment(undefined=jinja2.StrictUndefined)
+            try:
+                source = yaml.load(environment.from_string(stream.read()).render(self.data))
+            except UndefinedError as e:
+                raise UserError(f"error in '{path}': {e.message}") from e
             util.merge_into(self._data, source)
 
     def add_variable(self, key: str, value: str) -> None:
@@ -285,6 +286,7 @@ def __init__(self, context: Context, manifest_files: Sequence[Path]) -> None:
         }
         for manifest_file in manifest_files:
             with open(manifest_file, 'r') as f:
+                # TODO: load JSON manifests too (no reason why not, just use 'json.loads')
                 environment = jinja2.Environment(undefined=jinja2.StrictUndefined)
                 try:
                     manifest = yaml.load(environment.from_string(f.read()).render(context.data))