Support a default context in the kubecfg template

[negz]

Fixes #19.

[codecov]

Codecov Report

Merging #25 into master will increase coverage by 0.34%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   35.63%   35.97%   +0.34%     
==========================================
  Files           1        1              
  Lines         188      189       +1     
==========================================
+ Hits           67       68       +1     
  Misses        115      115              
  Partials        6        6

Impacted Files	Coverage Δ
cmd/kuberos/kuberos.go	35.97% <0%> (+0.34%)	⬆️

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