Allow callable references in sqlite-utils convert, closes #686

This allows users to pass just a callable reference like `r.parsedate`
instead of `r.parsedate(value)` to the convert command. The code now
detects when the input evaluates to a callable and uses it directly.

Examples that now work:
- sqlite-utils convert my.db table col r.parsedate
- sqlite-utils convert my.db table col json.loads --import json

sqlite_utils/utils.py

@@ -450,6 +450,10 @@ def progressbar(*args, **kwargs):
 
 def _compile_code(code, imports, variable="value"):
     globals = {"r": recipes, "recipes": recipes}
+    # Handle imports first so they're available for all approaches
+    for import_ in imports:
+        globals[import_.split(".")[0]] = __import__(import_)
+
     # If user defined a convert() function, return that
     try:
         exec(code, globals)
@@ -457,6 +461,15 @@ def _compile_code(code, imports, variable="value"):
     except (AttributeError, SyntaxError, NameError, KeyError, TypeError):
         pass
 
+    # Check if code is a direct callable reference
+    # e.g. "r.parsedate" instead of "r.parsedate(value)"
+    try:
+        fn = eval(code, globals)
+        if callable(fn):
+            return fn
+    except Exception:
+        pass
+
     # Try compiling their code as a function instead
     body_variants = [code]
     # If single line and no 'return', try adding the return
@@ -478,8 +491,6 @@ def _compile_code(code, imports, variable="value"):
     if code_o is None:
         raise SyntaxError("Could not compile code")
 
-    for import_ in imports:
-        globals[import_.split(".")[0]] = __import__(import_)
     exec(code_o, globals)
     return globals["fn"]