text: Title: [BUG] Jinja templating for FugueSQL is failing with string
  Body: **Minimal Code To Reproduce**
  
  ```python
  from fugue_sql import FugueSQLWorkflow
  data = [
      ["A", "2020-01-01", 10],
      ["A", "2020-01-02", None],
      ["A", "2020-01-03", 30],
      ["B", "2020-01-01", 20],
      ["B", "2020-01-02", None],
      ["B", "2020-01-03", 40]
  ]
  schema = "id:str,date:date,value:double"
  
  with FugueSQLWorkflow() as dag:
      df = dag.df(data, schema)
      x = "A"
      dag("""
          SELECT *
            FROM df
           WHERE id = {{x}}
           PRINT
      """)
  ```
  
  **Describe the bug**
  Inserting numeric variables work but string variables does not. 
  
  **Expected behavior**
  It should work 
  
  **Environment (please complete the following information):**
   - Backend: Pandas and Dask
   - Backend version: Fugue 0.4.9
   - Python version: 3.7
   - OS: linux/windows: Linux
  