Auto-decode columns declared as JSON in output modes (issue #579)

Columns declared with type JSON now decode automatically wherever nested
JSON is already supported (query/rows/search/memory output, and the
Python API), without needing --json-cols. Uses sqlite3's built-in
detect_types=PARSE_DECLTYPES + a registered JSON converter, so it works
transparently through joins and aliases. Flat formats (CSV/TSV/table/raw)
re-serialize decoded values back to JSON text instead of a Python repr.
Also adds "JSON" as a first-class column type for create()/add_column()/
transform(), and fixes column_affinity("JSON") so transform() doesn't
rewrite JSON columns as REAL.

docs/python-api.rst

@@ -1564,12 +1564,13 @@ You can add a new column to a table using the ``.add_column(col_name, col_type)`
     db.table("dogs").add_column("dob", datetime.date)
     db.table("dogs").add_column("image", "BLOB")
     db.table("dogs").add_column("website") # str by default
+    db.table("dogs").add_column("friends", "JSON")
 
 You can specify the ``col_type`` argument either using a SQLite type as a string, or by directly passing a Python type e.g. ``str`` or ``float``.
 
 The ``col_type`` is optional - if you omit it the type of ``TEXT`` will be used.
 
-SQLite types you can specify are ``"TEXT"``, ``"INTEGER"``, ``"FLOAT"``, ``"REAL"`` or ``"BLOB"``.
+SQLite types you can specify are ``"TEXT"``, ``"INTEGER"``, ``"FLOAT"``, ``"REAL"``, ``"BLOB"`` or ``"JSON"``. A column declared as ``"JSON"`` will be automatically decoded back into a Python object (rather than a JSON string) whenever it is read, both by the Python library and by the :ref:`CLI <cli_json_values>`.
 
 If you pass a Python type, it will be mapped to SQLite types as shown here::