some new future

[sunny0826]

• add clear cmd
• add filter future to ls cmd kubecm ls filter  #11
• add utest
• fix panic: runtime error: invalid memory address or nil pointer dereference #15
• update docs
• some optimisation

[codecov]

Codecov Report

Merging #16 (04e6ff4) into master (0986831) will increase coverage by 4.72%.
The diff coverage is 28.70%.

@@            Coverage Diff            @@
##           master     #16      +/-   ##
=========================================
+ Coverage    4.06%   8.78%   +4.72%     
=========================================
  Files          14      15       +1     
  Lines         688     785      +97     
=========================================
+ Hits           28      69      +41     
- Misses        659     707      +48     
- Partials        1       9       +8     

Flag	Coverage Δ
unittests	8.78% <28.70%> (+4.72%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/add.go	0.00% <0.00%> (ø)
cmd/cmd.go	0.00% <0.00%> (ø)
cmd/switch.go	0.00% <ø> (ø)
cmd/root.go	5.66% <11.11%> (+5.66%)	⬆️
cmd/clear.go	19.44% <19.44%> (ø)
cmd/list.go	14.63% <28.57%> (+14.63%)	⬆️
cmd/utils.go	14.21% <68.18%> (+12.07%)	⬆️

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