Add osflavor plugin

[eholic]

Add osflavor plugin automatically adding OS-flavored symbols to prompt.

[jorgebucaran]

Interesting! The introduction in your documentation could use some work, though. It's not clear what this does. For example, your README says that "it is" a prompt, but it really isn't. That's great, though, since I'm no longer accepting prompts (#119). The fact that it supports other well-known prompts is a good sign that you know what you're doing. I recommend reworking the documentation a bit and leaving this PR open for a while. Hopefully, other people will take a look or show interest in your project. Also, consider sharing it on /r/fishshell once you've improved the docs. Cheers!