split cpu and cpu list collect options

* Splitting cpu / cpus data output + update readme

Note : cpus and cpus can't be used in the same time if they are both using "metrics.cpu" to store data This leads to an error :
```
> ./sysinfo_influxdb -c="cpu,cpus" -P="metrics"

#0: metrics.cpu
| id    | user  | nice  | sys   | idle  | wait  | total |
| cpu   | 2665292   | 18800 | 713279    | 107185175 | 3379100   | 113963348 | 

#1: metrics.cpu
| id    | user  | nice  | sys   | idle  | wait  | total |
| cpu0  | 18446744073706886326  | 18446744073709532816  | 18446744073708838340  | 18446744073602366535  | 18446744073706172516  | 18446744073595588367  | 
| cpu1  | 2 | 0 | 2 | 96    | 0 | 100   | 
```
That's why I choose to use cpus and cpu group for both option :
```
> ./sysinfo_influxdb -c="cpu,cpus" -P="metrics"

#0: metrics.cpu
| id    | user  | nice  | sys   | idle  | wait  | total |
| cpu   | 13    | 0 | 11    | 772   | 0 | 796   | 

#1: metrics.cpus
| id    | user  | nice  | sys   | idle  | wait  | total |
| cpu0  | 5 | 0 | 7 | 89    | 0 | 101   | 
| cpu1  | 3 | 0 | 3 | 94    | 0 | 100   | 
| cpu2  | 1 | 0 | 1 | 96    | 0 | 98    | 
| cpu3  | 2 | 0 | 2 | 96    | 0 | 100   | 
| cpu4  | 1 | 0 | 0 | 98    | 0 | 99    | 
| cpu5  | 0 | 0 | 0 | 99    | 0 | 99    | 
| cpu6  | 0 | 0 | 0 | 100   | 0 | 100   | 
| cpu7  | 0 | 0 | 0 | 100   | 0 | 100   | 
```