Add vararg_callback utility function to beets.ui

Optparse doesn't support argparse nargs='+' style arguments.  This patch
adds a callback utility function that allows that idiom. The function is
taken from the page at
http://docs.python.org/2/library/optparse.html#callback-example-6-variable-arguments.

Here's an example of how to use it:

    from beets.ui import vararg_callback
    parser.add_option("-c", "--callback", dest="vararg_attr",
                      action="callback", callback=vararg_callback)

beets/ui/__init__.py

@@ -695,6 +695,33 @@ class SubcommandsOptionParser(optparse.OptionParser):
         return options, subcommand, suboptions, subargs
 
 
+# callback for an option with variable arguments
+# parser.add_option("-c", "--callback", dest="vararg_attr",
+#                   action="callback", callback=vararg_callback)
+
+def vararg_callback(option, opt_str, value, parser):
+    assert value is None
+    value = []
+
+    def floatable(str):
+        try:
+            float(str)
+            return True
+        except ValueError:
+            return False
+
+    for arg in parser.rargs:
+        # stop on --foo like options
+        if arg[:2] == "--" and len(arg) > 2:
+            break
+        # stop on -a, but not on -3 or -3.0
+        if arg[:1] == "-" and len(arg) > 1 and not floatable(arg):
+            break
+        value.append(arg)
+
+    del parser.rargs[:len(value)]
+    setattr(parser.values, option.dest, value)
+
 # The root parser and its main function.
 
 def _raw_main(args):

docs/changelog.rst

@@ -6,6 +6,10 @@ Changelog
 
 New features:
 
+* :ref:`add_subcommands`: Added ``optparse`` callback utility function, allowing
+  plugin developers to add options with ``action=callback`` and
+  ``callback=beets.ui.varargs_callback`` and a variable number of arguments.
+
 * :doc:`/plugins/duplicates`: The new ``keys`` option allows you to specify
   arbitrary fields over which to consider potential duplicates.
 

docs/dev/plugins.rst

@@ -44,6 +44,8 @@ containing your plugin).
 
 .. _virtualenv: http://pypi.python.org/pypi/virtualenv
 
+.. _add_subcommands:
+
 Add Commands to the CLI
 ^^^^^^^^^^^^^^^^^^^^^^^