--disable-subtract

Added `--disable-subtract` option. 

By disabling `subtract` functionality, we can improve the performance of the GrowthForecast worker drastically because

1. We can avoid N+1 queries issued at https://github.com/kazeburo/GrowthForecast/blob/c3bc89dd2d2f3ab01dbfd5a181e931cd15364761/lib/GrowthForecast/Worker.pm#L94-L105
2. We can avoid to have `sub` DS in RRD files, which results in reducing disk size and disk access to half
https://github.com/kazeburo/GrowthForecast/blob/c3bc89dd2d2f3ab01dbfd5a181e931cd15364761/lib/GrowthForecast/RRD.pm#L33

In my environments, I achieved following performance improvement to update 60,000 graphs

```
Before: 50 sec
After: 10 sec
```

I also prepared --disable-subtract option for `eg/alter_for_float_number.pl` and `eg/benchmark_rrd.pl`. 

Please note that I took care of that users can upgrade to use --disable-subtract without deleting RRD files and databases.

FYI: ?w=1 should be useful to see diff because many diffs are just indentation diffs. https://github.com/kazeburo/GrowthForecast/pull/50/files?w=1

EDIT: Because RRD itself has DERIVE mode which is actually same with subtract, we would be able to remove gmode (gauge or subtract or both) to make it simple in the future.