Convert an automatic registration into an option

Make registering a SpectrumList reader for any `data_loader` that reads
Spectrum1D objects optional, per the TODO in `specutils/io/registers.py`.

specutils/io/registers.py

@@ -26,7 +26,7 @@ def _astropy_has_priorities():
 
 
 def data_loader(label, identifier=None, dtype=Spectrum1D, extensions=None,
-                priority=0, force=False, verbose=False):
+                priority=0, force=False, opt_in_spectrumlist=True, verbose=False):
     """
     Wraps a function that can be added to an `~astropy.io.registry` for custom
     file reading.
@@ -51,6 +51,9 @@ def data_loader(label, identifier=None, dtype=Spectrum1D, extensions=None,
     force : bool, optional
         Whether to override any existing function if already present.
         Default is ``False``. Passed down to astropy registry.
+    opt_in_spectrumlist : bool, optional
+        Whether to register a SpectrumList reader for any data_loader that
+        reads Spectrum1D objects.  Default is ``True``.
     verbose : bool
         Print extra info.
 
@@ -105,10 +108,9 @@ def decorator(func):
         if verbose:
             print(f"Successfully loaded reader \"{label}\".")
 
-        # Automatically register a SpectrumList reader for any data_loader that
-        # reads Spectrum1D objects. TODO: it's possible that this
-        # functionality should be opt-in rather than automatic.
-        if dtype is Spectrum1D:
+        # Optionally register a SpectrumList reader for any data_loader that
+        # reads Spectrum1D objects.
+        if dtype is Spectrum1D and opt_in_spectrumlist:
             def load_spectrum_list(*args, **kwargs):
                 return SpectrumList([ func(*args, **kwargs) ])