Add a new configuration for livy.repl to always include additional files for interactive sessions

On our cluster, we want to include a hive-site.xml for every Livy interactive session. Right now, we have to specify the hive-site.xml in every interactive request, which is less than ideal.

We would like to add a configuration to automatically include a list of files in livy.repl. It would look like:

# Additional files livy will include when starting interactive sessions.
# It is equivalent to setting "--files" on every session request.
## livy.repl.additional.files = hdfs://localhost:8020/hive-site.xml