changed alert_command to critical_level_command
changed alert_command to critical_level_command

this change is made to differentiate the low-energy from the critical
level one better.

new options:
- critical_level_command="<some command>"
- critical_level_percentage=int
juliushaertl/i3pystatus,drwahl/i3pystatus,teto/i3pystatus,onkelpit/i3pystatus,richese/i3pystatus,m45t3r/i3pystatus,asmikhailov/i3pystatus,Arvedui/i3pystatus,Arvedui/i3pystatus,ncoop/i3pystatus,juliushaertl/i3pystatus,eBrnd/i3pystatus,richese/i3pystatus,enkore/i3pystatus,facetoe/i3pystatus,drwahl/i3pystatus,onkelpit/i3pystatus,Elder-of-Ozone/i3pystatus,fmarchenko/i3pystatus,asmikhailov/i3pystatus,Elder-of-Ozone/i3pystatus,m45t3r/i3pystatus,yang-ling/i3pystatus,schroeji/i3pystatus,ncoop/i3pystatus,eBrnd/i3pystatus,fmarchenko/i3pystatus,yang-ling/i3pystatus,schroeji/i3pystatus,facetoe/i3pystatus,teto/i3pystatus,enkore/i3pystatus