system and instances to add hostname / pluginname as columns in influxdb

[rplessl]

This patch adds:

• an enhanceable system for adding additional columns in influxdb
• added hostname column as one instance [1]
• added pluginname column as another instance of this system

[1]: enhancement of pull request https://github.com/hoonmin/influxdb-collectd-proxy/pull/16 from @JeroenBo (JeroenBo@0a0140e2a43cdfe7b8084ed9f171bbb98fe7524a)

[hoonmin]

Thank you for your PR and it looks good for me :-)

[rplessl]

Thanks!