nmbug-status: Slug the title when using it as an id

Also allow manual id overrides from the JSON config.  Sluggin avoids
errors like:

   Bad value '#Possible bugs' for attribute href on element a:
   Whitespace in fragment component. Use %20 in place of spaces.

from http://validator.w3.org.

I tried just quoting the titles (e.g. 'Possible%20bugs'), but that
didn't work (at least with Firefox 24.2.0).  Slugging avoids any
ambiguity over when the quotes are expanded in the client.  The specs
are unclear about quoting, saying only [1]:

  Value: Any string, with the following restrictions:

    must be at least one character long
    must not contain any space characters

[1]: http://dev.w3.org/html5/markup/global-attributes.html#common.attrs.id

devel/nmbug/nmbug-status

@@ -21,6 +21,7 @@ except ImportError:  # Python 2
 import json
 import argparse
 import os
+import re
 import sys
 import subprocess
 
@@ -168,16 +169,20 @@ class Page (object):
 
 
 class HtmlPage (Page):
+    _slug_regexp = re.compile('\W+')
+
     def _write_header(self, views, stream):
         super(HtmlPage, self)._write_header(views=views, stream=stream)
         stream.write('<ul>\n')
         for view in views:
+            if 'id' not in view:
+                view['id'] = self._slug(view['title'])
             stream.write(
-                '<li><a href="#{title}">{title}</a></li>\n'.format(**view))
+                '<li><a href="#{id}">{title}</a></li>\n'.format(**view))
         stream.write('</ul>\n')
 
     def _write_view_header(self, view, stream):
-        stream.write('<h3 id="{title}">{title}</h3>\n'.format(**view))
+        stream.write('<h3 id="{id}">{title}</h3>\n'.format(**view))
         if 'comment' in view:
             stream.write(view['comment'])
             stream.write('\n')
@@ -224,6 +229,9 @@ class HtmlPage (Page):
                 ).format(**d)
         return (running_data, display_data)
 
+    def _slug(self, string):
+        return self._slug_regexp.sub('-', string)
+
 
 _PAGES['text'] = Page()
 _PAGES['html'] = HtmlPage(