Don't error out on missing config entry

[xeruf]

I commented out a config option I do not need right now, promptly causing an error:

Traceback (most recent call last):
  File "/home/janek/data/1-projects/1-personal/lazyblorg/./lazyblorg.py", line 606, in <module>
    statistics_list = lazyblorg.generate_output(generate, marked_for_feed, increment_version)
  File "/home/janek/data/1-projects/1-personal/lazyblorg/./lazyblorg.py", line 180, in generate_output
    return htmlizer.run()  # FIXXME: return value?
  File "/home/janek/data/1-projects/1-personal/lazyblorg/lib/htmlizer.py", line 214, in run
    stats_generated_persistent, stats_generated_tags = self._generate_pages_for_tags_persistent_temporal(tags)
  File "/home/janek/data/1-projects/1-personal/lazyblorg/lib/htmlizer.py", line 378, in _generate_pages_for_tags_persistent_temporal
    htmlfilename, orgfilename, htmlcontent = self._generate_page(config.TEMPORAL, entry)
  File "/home/janek/data/1-projects/1-personal/lazyblorg/lib/htmlizer.py", line 467, in _generate_page
    htmlcontent = self.sanitize_internal_links(self._replace_general_article_placeholders(entry, htmlcontent))
  File "/home/janek/data/1-projects/1-personal/lazyblorg/lib/htmlizer.py", line 2131, in _replace_general_article_placeholders
    content = self._replace_general_blog_placeholders(template)
  File "/home/janek/data/1-projects/1-personal/lazyblorg/lib/htmlizer.py", line 2091, in _replace_general_blog_placeholders
    content = content.replace('#TOP-TAG-LIST#', self._generate_top_tag_list())  # FIXXME: generate only once for performance reasons?
  File "/home/janek/data/1-projects/1-personal/lazyblorg/lib/htmlizer.py", line 2181, in _generate_top_tag_list
    if tag not in config.IGNORE_FOR_TOP_TAGS and \
AttributeError: module 'config' has no attribute 'IGNORE_FOR_TOP_TAGS'

Would be nice if lazyblorg would gracefully use an empty default.

[xeruf]

For example, omitting my DISQUS_NAME should disable the commenting feature.

[novoid]

Of course, you're right (again).

Feel free to adapt and hand in a pull request. It's not part of my set of requirements so I won't invest any time for that.