Keep certain files when cleaning output; fix #574

If DELETE_OUTPUT_DIRECTORY is set to True, all files and directories are
deleted from the output directory. There are, however, several reasons
one might want to retain certain files/directories and avoid their
deletion from the output directory. One such use case is version control
system data: a versioned output directory can facilitate deployment via
Heroku and/or allow the user to easily revert to a prior version of the
site without having to rely on regeneration via Pelican.

This change introduces the OUTPUT_RETENTION setting, a tuple of
filenames that will be preserved when the clean_output_dir function in
pelican.utils is run. Setting OUTPUT_RETENTION = (".hg", ".git") would,
for example, prevent the relevant VCS data from being deleted when the
output directory is cleaned.

pelican/settings.py

@@ -54,6 +54,7 @@ DEFAULT_CONFIG = {
     'NEWEST_FIRST_ARCHIVES': True,
     'REVERSE_CATEGORY_ORDER': False,
     'DELETE_OUTPUT_DIRECTORY': False,
+    'OUTPUT_RETENTION': (),
     'ARTICLE_URL': '{slug}.html',
     'ARTICLE_SAVE_AS': '{slug}.html',
     'ARTICLE_LANG_URL': '{slug}-{lang}.html',