This is a combination of 13 commits:
1. New signal for registering custom generators
2. New plugin: pelican.plugins.sitemap
3. pelican.plugins.sitemap: more settings
4. pelican.plugins.sitemap: translations are indexed
5. pelican.plugins.sitemap: added documentation
6. pelican.plugins.sitemap: added XML DTD & W3C dates
7. pelican.plugins.sitemap: removed a <changefreq> bug
8. the `get_generators` can now return a tuple
9. pelican.plugins.sitemap: cleaned the code
10. pelican.plugin.sitemap: settings changes
11. sitemap plugin: improved configuration & documentation
12. sitemap plugin: :set spell
13. sitemap plugin: removed useless whitespaces
Most of the references to code and settings in the docs were wrapped
in single tickmarks (`), while reStructuredText syntax actually calls
for double tickmarks for inline literals, which are normally rendered
as monospaced text with spaces preserved. Converted the relevant
instances to inline literals, along with some other minor fixes.
