?_labels= and ?_label=COL to expand foreign keys in JSON/CSV

These new querystring arguments can be used to request expanded foreign keys
in both JSON and CSV formats.

?_labels=on turns on expansions for ALL foreign key columns

?_label=COLUMN1&_label=COLUMN2 can be used to pick specific columns to expand

e.g. `Street_Tree_List.json?_label=qSpecies&_label=qLegalStatus`

    {
        "rowid": 233,
        "TreeID": 121240,
        "qLegalStatus": {
            "value" 2,
            "label": "Private"
        }
        "qSpecies": {
            "value": 16,
            "label": "Sycamore"
        }
        "qAddress": "91 Commonwealth Ave",
        ...
    }

The labels option also works for the HTML and CSV views.

HTML defaults to `?_labels=on`, so if you pass `?_labels=off` you can disable
foreign key expansion entirely - or you can use `?_label=COLUMN` to request
just specific columns.

If you expand labels on CSV you get additional columns in the output:

`/Street_Tree_List.csv?_label=qLegalStatus`

    rowid,TreeID,qLegalStatus,qLegalStatus_label...
    1,141565,1,Permitted Site...
    2,232565,2,Undocumented...

I also refactored the existing foreign key expansion code.

Closes #233. Refs #266.

datasette/utils.py

@@ -1,4 +1,5 @@
 from contextlib import contextmanager
+from collections import OrderedDict
 import base64
 import hashlib
 import imp
@@ -802,6 +803,23 @@ def path_with_format(request, format, extra_qs=None):
     return path
 
 
+class CustomRow(OrderedDict):
+    # Loose imitation of sqlite3.Row which offers
+    # both index-based AND key-based lookups
+    def __init__(self, columns):
+        self.columns = columns
+
+    def __getitem__(self, key):
+        if isinstance(key, int):
+            return super().__getitem__(self.columns[key])
+        else:
+            return super().__getitem__(key)
+
+    def __iter__(self):
+        for column in self.columns:
+            yield self[column]
+
+
 def value_as_boolean(value):
     if value.lower() not in ('on', 'off', 'true', 'false', '1', '0'):
         raise ValueAsBooleanError