The documentation doesn't make it very clear how to prevent certain
pages from being generated, such as the Authors, Tags, and Categories
collection pages. This change makes it slightly more obvious how to
prevent these pages from being generated. Fixes #940.

I think the author list and tag list are so common that they should be
listed explicitly in the settings.

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.

Generate context objects in read_file()

Useful for altering static output paths when you don't want to encode
extra metadata in the static file's source path.