add 'help' command everywhere

[vetinari]

so we can also call e.g.

    zmon alert help

instead of

    zmon alert --help

[codecov[bot]]

Codecov Report

Merging #57 into master will decrease coverage by 0.45%. The diff coverage is 68%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   74.58%   74.13%   -0.46%     
==========================================
  Files          19       18       -1     
  Lines        1027     1040      +13     
==========================================
+ Hits          766      771       +5     
- Misses        261      269       +8

Impacted Files	Coverage Δ
zmon_cli/cmds/entity.py	45.71% <66.66%> (+0.93%)	:arrow_up:
zmon_cli/cmds/grafana.py	54.54% <66.66%> (+1.21%)	:arrow_up:
zmon_cli/cmds/group.py	41.77% <66.66%> (+0.98%)	:arrow_up:
zmon_cli/cmds/check.py	82.89% <66.66%> (-0.67%)	:arrow_down:
zmon_cli/cmds/dashboard.py	47.61% <66.66%> (+1.46%)	:arrow_up:
zmon_cli/cmds/alert.py	64.51% <66.66%> (+0.07%)	:arrow_up:
zmon_cli/cmds/downtime.py	57.14% <66.66%> (+1.14%)	:arrow_up:
zmon_cli/cmds/command.py	90.56% <75%> (-1.28%)	:arrow_down:
zmon_cli/cmds/__init__.py	100% <0%> (ø)	:arrow_up:
zmon_cli/__init__.py

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[vetinari]

:+1:

[mohabusama]

👍