Allows users to declare custom aliases

[nkakouros]

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cc: @mbland

This PR adds a GO_ALIAS_CMDS_EXTRA array that the user can define to add their own aliases. My use case was that I grew tired of typing go vim and failing all the time (instead of go run vim).

I also added the ability for "go-script" aliases to be expanded if they are bash aliases.

[coveralls]

Coverage increased (+0.01%) to 95.01% when pulling 3849154 on tterranigma:extra_aliases into 292e080 on mbland:master.