windows support

[hfn92]

I'm sometimes forced to use windows and I'd still like to be able to use gh.nvim

I've identified two issues under windows.

1. temp dir - /tmp is not available under window. gh.nvim can't show diffs because it cant pull the source files. I don't know what a sensible temp dir is for windows. But I think being able to configure the the temp dir would solve the issue.

2. some gh cli invokations fail. Not sure why. But the body is cut off if a comment contains "\n":
   '{"message":"Validation Failed","errors":[{"resource":"IssueComment","code":"unprocessable","field":"data","message":"Body cannot be blank"}],"documentation_url":https://docs.github.com/enterprise-server@3.9/rest/reference/issues#create-an-issue-comment}gh: Validation Failed (HTTP 422)\n'
   I was able to fix the by changing the way vim.fn.system is called

   -- from this
    local cmd = string.format([[gh api -X POST /repos/{owner}/{repo}/issues/%d/comments -f body=%s]], number, body)
    -- to this
    local cmd = {
      "gh",
      "api",
      "-X",
      "POST",
      string.format([[/repos/{owner}/{repo}/issues/%s/comments]], number),
      "-f",
      string.format([[body=%s]], body:sub(2, #body - 1)), -- remove " from the beginning and end
    }

I fixed what didn't work for me on this branch hfn92@470ec8f
I'm unsure about side effects 😟 But what do you think. Are those reasonable changes?