Allows _GO_SCRIPTS_DIR to be equal to _GO_ROOTDIR

[nkakouros]

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cc: @mbland

What the title says. This adds flexibility to the structure of projects using go-script-bash.

I had encountered a need to set _GO_ROOTDIR to _GO_SCRIPTS_DIR while working on bash-headstart.

[coveralls]

Coverage decreased (-0.05%) to 94.943% when pulling 21e0aaf2870acfac673cb3d5b38ff9379fac22ae on tterranigma:rootdir-script-dir into 292e0803074eb538ab8633a657c53d3a975ba922 on mbland:master.