Supports multiple command script sources

[nkakouros]

• I have reviewed the contributor guidelines.
• I have reviewed the code of conduct.
• Per GitHub's Terms of Service, I am aware that I license my
  contribution under the same terms as this project's license, and
  that I have the right to license my contribution under those terms.

cc: @mbland

I added support for multiple command script sources by making _GO_SCRIPTS_DIR an array. This allows for flexible project structures.

My use case

I am using go-script-bash in many project of mine. I have build some scripts and a folder structure that I use in all my projects. But, while working on a project, I add features, fix bugs, etc on this common 'skeleton'. Then, I have to copy stuff around to bring all my projects on par. I would like to have this skeleton as a separate project that I can add as a git submodule. For this to happen, it is needed that go-script-bash supports multiple command script sources. In this case, I would be able to load both the commands from the "skeleton" project and each separate project that is build on top of the "skeleton".

[nkakouros]

@mbland I realize you are currently lacking time to go through the issues/PRs, it would be nice to have some feedback, though, whenever you find the time.

[coveralls]

Coverage increased (+0.09%) to 95.087% when pulling 9814499 on nkakouros-forks:multiple-script-dirs into 292e080 on mbland:master.

[nkakouros]

The error on windows is a strange one. On linux instances, the result of the failing test is scripts-2 scripts. I have tried with bash 3.2, 4.3, 4.4 and 5.0 on Arch and Ubuntu 18.04. On Windows, however it is the other way around. Judging by the tests on macos, this holds true there too. I also tried with Centos7 and it follows Windows' behavior.

I tried to find the cause of this and my finding was that compgen behaves differently. On linux, running compgen -f -- on the prompt of bash --norc --noprofile, does not order the returned values alphabetically. On windows, it does. Adding -o nosort does not change the result.

I don't know how to debug this further.

As for the failing macos test, I don't have access to a macos installation to try to debug it.