Add a 'set default' sub-operation to 'alter column'

[andrew-farries]

Allow 'alter column' operations to set the default value on a column:

{
  "name": "02_change_default",
  "operations": [
    {
      "alter_column": {
        "table": "events",
        "column": "name",
        "default": "'new default value'"
      }
    }
  ]
}

This is a versioned migration so the column to which the default is applied is duplicated and backfilled according to the up and down SQL supplied with the 'alter column' operation. up and down default to a simple copy of the field between new and old columns.

Fixes #327