allow assigning variables to tasks at run time via CLI

using a similar syntax than setting env variables to command in bash,
but used right after the task:

```bash
task print MESSAGE=Hello!
```

closes #33

Taskfile.yml

@@ -34,6 +34,7 @@ test:
   desc: Runs test suite
   deps: [install]
   cmds:
+    - go test ./args
     - go test
 
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