plugin/photos: add support to skip gallery

Control whether to exclude from the build some photo gallery or them all.

PELICAN_PHOTO_EXCLUDE and PELICAN_PHOTO_EXCLUDEALL can be set in the
environment. The former is a whitespace separated list of directory
_basename_s, the latter is a binary 0 or 1.

Note: when excluding galleries, pelican will still generate html files
accounting for photos, so it is possible to defer processing images and
installing them separately from the html.

i.e.

  PELICAN_PHOTO_EXCLUDE="gallery1 gallery9"

or

  PELICAN_PHOTO_EXCLUDEALL="1"
2 files changed