Allows custom plugins to pop up during completion with standalone projects

[nkakouros]

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cc: @mbland

When setting _GO_STANDALONE, custom plugins are not returned with tab completion. This is counter-intuitive IMHO. Plugins are super convenient to organize code and share it among projects. There is no reason really for such custom code to not be returned as suggested commands to the user, since the purpose of that custom code is 99.9% to be used by the user.

[coveralls]

Coverage increased (+0.004%) to 95.001% when pulling d91b75e9de4993f88331ad8fffe205c4cb366389 on tterranigma:standalone-plugins into 292e0803074eb538ab8633a657c53d3a975ba922 on mbland:master.