Allow static files in Collections

Allow StaticFiles in Collections alongside Documents. Attempts to fix #2555, #2598. Can also be modified a bit to accommodate #2592. Builds on #2603.

Please test!
Is this a bit too complex?

```ruby
# Returns the source file path relative to the site source
def relative_path
  @relative_path ||= File.join(*[@dir, @name].compact)
end

# Obtain destination path.
#
# dest - The String path to the destination dir.
#
# Returns destination file path.
def destination(dest)
  File.join(*[dest, collection ? relative_path.gsub(/^\_/, '') : relative_path].compact)
end
```