Add completion configuration for non-Homebrew fish

[gromgit]

Not everyone uses Homebrew fish, especially on Linux. Tested on Ubuntu 18.04 LTS.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

[dawidd6]

set -g -x could be set -gx. Just a suggestion. Otherwise LGTM.

[gromgit]

set -g -x could be set -gx. Just a suggestion. Otherwise LGTM.

Thanks, @dawidd6. I've made the corresponding edits.

[MikeMcQuaid]

Thanks @gromgit!

[jameshome]

I ran into this again installing x86 brew in a Rosetta iTerm session after installing universal fish independently.

Error: Failed to link all completions, docs and manpages:
Permission denied @ rb_file_s_symlink - (../../../Homebrew/completions/fish/brew.fish, /usr/local/share/fish/vendor_completions.d/brew.fish)

I just copied the completions over manually and it works fine. Y'all definitely have bigger fish to fry, but I wanted to mention it.