Allow "merging" rclone options

[chazlarson]

It would be useful to be able to use a default list of rclone options, then add to or override these options on an individual syncer/mover/deduper level.

This would prevent having to duplicate the entire list of rclone options into each syncer/mover/deduper only to change one or two.

For example, say you want to use a different backup-dir on different syncers. Something like this would be useful and convenient:

rclone:
  ...
  global_params:
    default:
      sync:
        - '--fast-list'
        - '--tpslimit-burst=50'
        ...etc

syncer:

  - name: td_1 sync
    enabled: true
    source_remote: 'td_1:/stuff'
    remotes:
      sync:
        - 'td_1_mirror:/stuff'
    rclone_params:
      global_sync: default
      sync:
        - '--backup-dir=td_1_mirror:backup'

  - name: td_2 sync
    enabled: true
    source_remote: 'td_2:/stuff'
    remotes:
      sync:
        - 'td_2_mirror:/stuff'
    rclone_params:
      global_sync: default
      sync:
        - '--backup-dir=td_2_mirror:backup'

Each syncer would inherit the global set of flags, and then add on unique one. This prevents having to duplicate a common set of flags in every syncer entry.