More robust plugin list in conf

A single plugin name or an absence of it, with or without comma,
is now properly handled.

Related: #18

pubs/config/spec.py

@@ -68,8 +68,9 @@ pages     = string(default='')
 
 
 [plugins]
-# comma-separated list of the plugins to load
+# Comma-separated list of the plugins to load.
-active = list(default=list())
+# The only current available plugin is alias.
+active = force_list(default=list('alias'))
 
 [[alias]]
 # new subcommands can be defined, e.g.:

pubs/plugins.py

@@ -35,7 +35,8 @@ def load_plugins(conf, ui):
     PapersPlugin subclasses desired.
     """
     for name in conf['plugins']['active']:
-        modname = '%s.%s.%s.%s' % ('pubs', PLUGIN_NAMESPACE, name, name)
+        if len(name) > 0:
+            modname = '{}.{}.{}.{}'.format('pubs', PLUGIN_NAMESPACE, name, name)
             try:
                 namespace = importlib.import_module(modname)
             except ImportError as exc: