Add siac renter commands for listing upload and download queues

Created by: VoidingWarranties

siac renter uploads lists only files that are currently uploading.

siac renter downloads [--history/-H] lists only files that are currently downloading. The --history flag shows previously downloaded files in addition to the currently downloading files. There is no --history flag for siac renter uploads because the backend does not yet support this.

I also moved all command.AddCommand(subCommand) into init() functions in the respective command's file. This way the main function doesn't need to worry about adding all of the subcommands.