Add ROOT_DIR and TASKFILE_DIR special variables

Closes #215

testdata/special_vars/Taskfile.yml

@@ -0,0 +1,18 @@
+version: '3'
+
+includes:
+  included:
+    taskfile: ./included
+    dir: ./included
+
+tasks:
+  default:
+    cmds:
+      - task: print
+      - task: included:print
+
+  print:
+    cmds:
+      - echo root/TASK={{.TASK}}
+      - echo root/ROOT_DIR={{.ROOT_DIR}}
+      - echo root/TASKFILE_DIR={{.TASKFILE_DIR}}

testdata/special_vars/included/Taskfile.yml

@@ -0,0 +1,8 @@
+version: '3'
+
+tasks:
+  print:
+    cmds:
+      - echo included/TASK={{.TASK}}
+      - echo included/ROOT_DIR={{.ROOT_DIR}}
+      - echo included/TASKFILE_DIR={{.TASKFILE_DIR}}