Support an "everything" config attribute to merge all ressources into a single feed

[whysthatso]

as mentioned by @joemasilotti in #7 it would be very neat to have a config option that generates

• a single feed.xml endpoint
• including all posts
• including all custom collections

feed:
  include_all: true (default to false)

it should probably live along the current option to have a feed/.xml endpoint

[whysthatso]

an alternative more elaborate version could also give the option of a multifeed. In this example the site has 2 custom collections coll_1 and coll_2. the name of the feed could be default multifeed.xml or have a custom name. so i could overwrite feed.xml to become my multifeed. or have feed.xml behave like it currently does.

feed:
  include_all: true (default to false)
  multifeed:
    collections:
      - posts
      - coll_2
     path: multifeed.xml (default)

one could suggest to even have multiple feeds, but i haven't really thought it all the way through, so comments are welcome.

These are possibly related issues:
bridgetownrb/bridgetown#730
bridgetownrb/bridgetown#384