Make optional cpu usage per core

[monicasarbu]

Duplicates https://github.com/elastic/topbeat/issues/134

In the PR #130, the cpu usage info per core is exported by Topbeat. For each core the following data are exported:

#cpu0.idle      7,493,465
#cpu0.iowait        0
#cpu0.irq       0
#cpu0.nice      0
#cpu0.softirq       0
#cpu0.steal     0
#cpu0.system        950,044
#cpu0.system_p      0.05
#cpu0.user      1,414,674
#cpu0.user_p        0.13

• make optional exporting cpu usage per core Group all cpu usage per core statistics under cpus #496
• update changelog Update changelog #543
• group all info under cpus Group all cpu usage per core statistics under cpus #496