[Jekyll] Add support for globs in the keep_files option

## Summary It would be great if users could use globs in the "keep_files" configuration option. ## Motivation As of now the user needs to know and explicitly list every single file to be preserved. This is verbose and inflexible, e.g. they can't keep all files with the given extension or a prefix. ## Guide-level explanation Copying this snippet from the "include" configuration option docs and adding it to the "keep_files" docs will suffice: > This configuration option supports Ruby's File.fnmatch filename globbing patterns to match multiple entries to include, refer the exclude configuration option for more information. ## Reference-level explanation Copy the implementation of either "include" or "exclude". ## Drawbacks It may break backward compatibility in edge cases where an existing configuration lists files that include glob special characters. ## Unresolved Questions What is life?