Support semicolon-separated author/tag lists.

Idea borrowed from Docutils.  This allows one to write author lists in
lastname,firstname format.  The code change also means that readers with
fancy metadata that can natively represent lists (e.g. Docutils itself,
or MD-Yaml) don't have to merge 'em back together for process_metadata's
sake.

pelican/readers.py

@@ -27,11 +27,25 @@ from pelican import signals
 from pelican.contents import Page, Category, Tag, Author
 from pelican.utils import get_date, pelican_open, FileStampDataCacher, SafeDatetime, posixize_path
 
+def ensure_metadata_list(text):
+    """Canonicalize the format of a list of authors or tags.  This works
+       the same way as Docutils' "authors" field: if it's already a list,
+       those boundaries are preserved; otherwise, it must be a string;
+       if the string contains semicolons, it is split on semicolons;
+       otherwise, it is split on commas.  This allows you to write
+       author lists in either "Jane Doe, John Doe" or "Doe, Jane; Doe, John"
+       format.
 
-def strip_split(text, sep=','):
-    """Return a list of stripped, non-empty substrings, delimited by sep."""
-    items = [x.strip() for x in text.split(sep)]
-    return [x for x in items if x]
+       Regardless, all list items undergo .strip() before returning, and
+       empty items are discarded.
+    """
+    if isinstance(text, six.text_type):
+        if ';' in text:
+            text = text.split(';')
+        else:
+            text = text.split(',')
+
+    return [v for v in (w.strip() for w in text) if v]
 
 
 # Metadata processors have no way to discard an unwanted value, so we have
@@ -50,13 +64,16 @@ def _process_if_nonempty(processor, name, settings):
 
 
 METADATA_PROCESSORS = {
-    'tags': lambda x, y: [Tag(tag, y) for tag in strip_split(x)] or _DISCARD,
+    'tags': lambda x, y: ([Tag(tag, y) for tag in ensure_metadata_list(x)]
+                          or _DISCARD),
     'date': lambda x, y: get_date(x.replace('_', ' ')),
     'modified': lambda x, y: get_date(x),
     'status': lambda x, y: x.strip() or _DISCARD,
     'category': lambda x, y: _process_if_nonempty(Category, x, y),
     'author': lambda x, y: _process_if_nonempty(Author, x, y),
-    'authors': lambda x, y: [Author(a, y) for a in strip_split(x)] or _DISCARD,
+    'authors': lambda x, y: ([Author(author, y)
+                              for author in ensure_metadata_list(x)]
+                             or _DISCARD),
     'slug': lambda x, y: x.strip() or _DISCARD,
 }
 
@@ -179,7 +196,6 @@ class RstReader(BaseReader):
                 elif element.tagname == 'authors':  # author list
                     name = element.tagname
                     value = [element.astext() for element in element.children]
-                    value = ','.join(value) # METADATA_PROCESSORS expects a string
                 else:  # standard fields (e.g. address)
                     name = element.tagname
                     value = element.astext()