feat: support kubectl-style plugin

[zyy17]

What's the feature

We can use the kubectl-sytle plugin now. For example, if we put the custom plugin in the /usr/local/bin, like gtctl-foo, we can use it like this:

GTCTL_PLUGIN_PATHS=/usr/local/bin gtctl foo 1 2 3

The gtctl will search the plugin from the GTCTL_PLUGIN_PATHS(separate by :) and run it.

Related issue: #24.

[codecov-commenter]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (5e2fb15) 37.79% compared to head (889c3c3) 38.18%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #175      +/-   ##
===========================================
+ Coverage    37.79%   38.18%   +0.38%     
===========================================
  Files           14       15       +1     
  Lines         1233     1286      +53     
===========================================
+ Hits           466      491      +25     
- Misses         668      690      +22     
- Partials        99      105       +6     

Files	Coverage Δ
pkg/plugins/manager.go	47.16% <47.16%> (ø)

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[shawnh2]

lgtm