Split git-issue.sh into multiple files

With the main script file nearing 1200 lines of code --and my [GSoC version](https://github.com/eellak/gsoc2019-git-issue/blob/new_attributes/git-issue.sh) at 1500-- and getting a bit unwieldy, I was considering splitting out some of the functions to a secondary source file(for example the ones concerning _GitHub/GitLab_ `import/export`). Would that be OK, or would it be preferable to keep it to the single file, e.g for easier installation?