feat: support .py and .sh extensions

[Finkregh]

Would allow installing something like https://github.com/babarot/httpstat

[Finkregh]

based on changes in #110
@autarch please have a look :)

[autarch]

This looks good % a few small things.

[autarch]

Hi, thanks for your PR! I'm pretty finicky about my projects (see this blog post for details), so I rarely merge a PR as-is. I can move forward on your PR in one of two ways:

1. I check it out locally, fiddle with it as needed, merge it locally, and simply close this PR. This will preserve at least one commit with your name on it, but the PR will show up as closed in your GitHub stats.
2. If you enable me to push directly to your PR branch (which is the default when you make a PR), I can do my fiddling, then force push to your PR branch and merge the resulting PR. Again, this will preserve at least one commit with your name on it, but you also get credit for the PR merge in your GitHub stats. The only downside is that I will be force pushing directly to your PR branch. Note that this will not work if the PR branch is named master. GitHub doesn't allow me to push to the default branch of your fork.

Please let me know which approach you'd prefer. If I don't hear from you before I get around to working on this PR I'll go with option 1.

Thanks again for your contribution!

[Finkregh]

2 looks good, feel free to play around :)

[autarch]

I think this is a useful addition, but I realized it won't let you install babarot/httpstat, because that project doesn't actually publish GitHub releases. You can't use ubi to install arbitrary files from a remote Git repo (you can just use git for that).

[Finkregh]

Thanks!