main,modes: add extra CQL clause option

[psarna]

The extra-clause option allows appending a custom CQL clause
to all statements in the chosen workload. Example usage:

$> ./scylla-bench -mode read -workload sequential -extra-clause "using timeout 10ms"
Results
Time (avg):	 48.997763214s
Total ops:	 49748
Total rows:	 49748
Total errors:	 50252
Operations/s:	 1015.0848019658131
Rows/s:		 1015.0848019658131
Latency:
  max:		 15.040511ms
  99.9th:	 11.468799ms
  99th:		 10.027007ms
  95th:		 8.945663ms
  90th:		 8.388607ms
  median:	 6.062079ms
  mean:		 6.225544ms

[nyh]

This addition seems reasonable to me, but I'd love to see someone more familiar than me with scylla-bench review this before I go ahead and commit it.
Also, maybe this new option should be documented in README.md, which seems to document most (all?) of the existing options?