Issue #519: Allow includes to be optional

testdata/includes_optional/.gitignore

@@ -0,0 +1 @@
+*.txt

testdata/includes_optional/Taskfile.yml

@@ -0,0 +1,11 @@
+version: '3'
+
+includes:
+  included:
+    taskfile: TaskfileOptional.yml
+    optional: true
+
+tasks:
+  default:
+    cmds:
+      - echo "called_dep" > called_dep.txt

testdata/includes_optional_explicit_false/Taskfile.yml

@@ -0,0 +1,11 @@
+version: '3'
+
+includes:
+  included:
+    taskfile: TaskfileOptional.yml
+    optional: false
+
+tasks:
+  default:
+    cmds:
+      - echo "Hello, world!"

testdata/includes_optional_implicit_false/Taskfile.yml

@@ -0,0 +1,9 @@
+version: '3'
+
+includes:
+  included: TaskfileOptional.yml
+
+tasks:
+  default:
+    cmds:
+      - echo "Hello, world!"