text: Title: Schema generation enhancements
  Body: Piccolo now has a command for generating a schema from an existing database.
  
  ```bash
  piccolo schema generate
  ```
  
  The current implementation doesn't cover every possible edge case and database feature, so there's room for improvement. For example:
  
  ## Column defaults
  
  Getting the defaults for a column, and reflecting them in the Piccolo column. For example, if the column has a default of `1` in the database, we want that to be reflected in the Piccolo schema as:
  
  ```python
  class MyTable(Table):
      my_column = Integer(default=1)
  ```
  
  ## On Delete / On Update
  
  The database values aren't currently reflected in the ``ForeignKey`` column definitions.
  
  ```python
  class MyTable(Table):
      my_column = ForeignKey(on_delete=OnDelete.cascade)
  ```
  
  ## Decimal precision
  
  The precision of a decimal column in the database currently isn't reflected in the ``Decimal`` column definitions.
  
  ```python
  class MyTable(Table):
      my_column = Decimal(digits=(5,2))
  ```
  
  --------
  
  It's a fairly beginner friendly feature to work on, because even though the code is fairly advanced, it's completely self contained, and doesn't require extensive knowledge of the rest of Piccolo.
  