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

args/args.go

@@ -0,0 +1,34 @@
+package args
+
+import (
+	"errors"
+	"strings"
+
+	"github.com/go-task/task"
+)
+
+var (
+	ErrVariableWithoutTask = errors.New("task: variable given before any task")
+)
+
+func Parse(args ...string) ([]task.Call, error) {
+	var calls []task.Call
+
+	for _, arg := range args {
+		if !strings.Contains(arg, "=") {
+			calls = append(calls, task.Call{Task: arg})
+			continue
+		}
+		if len(calls) < 1 {
+			return nil, ErrVariableWithoutTask
+		}
+
+		if calls[len(calls)-1].Vars == nil {
+			calls[len(calls)-1].Vars = make(task.Vars)
+		}
+
+		pair := strings.SplitN(arg, "=", 2)
+		calls[len(calls)-1].Vars[pair[0]] = task.Var{Static: pair[1]}
+	}
+	return calls, nil
+}