sqlite-utils convert command and db[table].convert(...) method

Closes #251, closes #302.

tests/test_docs.py

@@ -1,10 +1,12 @@
-from sqlite_utils import cli
+from click.testing import CliRunner
+from sqlite_utils import cli, recipes
 from pathlib import Path
 import pytest
 import re
 
 docs_path = Path(__file__).parent.parent / "docs"
 commands_re = re.compile(r"(?:\$ |    )sqlite-utils (\S+) ")
+recipes_re = re.compile(r"r\.(\w+)\(")
 
 
 @pytest.fixture(scope="session")
@@ -17,11 +19,36 @@ def documented_commands():
     }
 
 
+@pytest.fixture(scope="session")
+def documented_recipes():
+    rst = (docs_path / "cli.rst").read_text()
+    return set(recipes_re.findall(rst))
+
+
 @pytest.mark.parametrize("command", cli.cli.commands.keys())
 def test_commands_are_documented(documented_commands, command):
     assert command in documented_commands
 
 
 @pytest.mark.parametrize("command", cli.cli.commands.values())
-def test_commands_have_docstrings(command):
-    assert command.__doc__, "{} is missing a docstring".format(command)
+def test_commands_have_help(command):
+    assert command.help, "{} is missing its help".format(command)
+
+
+def test_convert_help():
+    result = CliRunner().invoke(cli.cli, ["convert", "--help"])
+    assert result.exit_code == 0
+    for expected in (
+        "r.jsonsplit(value, ",
+        "r.parsedate(value, ",
+        "r.parsedatetime(value, ",
+    ):
+        assert expected in result.output
+
+
+@pytest.mark.parametrize(
+    "recipe",
+    [n for n in dir(recipes) if not n.startswith("_") and n not in ("json", "parser")],
+)
+def test_recipes_are_documented(documented_recipes, recipe):
+    assert recipe in documented_recipes