Append Go file type when creating temporary file

[mocheryl]

A lot of scripts filter files based on their name to make sure they
operate only on the appropriate content. By appending a Go file type to
a temporary file when running a formatter, we add support for a broader
range of Go formating tools.

Fixes #208

[arp242]

Thanks! I think this should be fine, but would be good to test on Windows, too. Just to be sure.

I don't think that any of the regular Go maintainers use Windows – I don't have a Windows machine anyway – but I'll look at running the tests on AppVeyor too, so we can test it in the CI.

[mocheryl]

I don't have a Windows machine anyway

Neither do I (tried on Linux using neovim), but if there's no other way, I'll see if I can procure one from somewhere.

If it doesn't work, then another way would be to pipe the buffer content to standard input of the formatter, but that would require be a bit more code change.

[arp242]

Well, I acquired a Windows VM from modern.ie. Setting up AppVeyor is going to be a lot more work than I thought.

I'll test tomorrow. It was mentioned in another PR a while ago that tempname() behaves quite different on Windows than it does on Linux/macOS, which is why it would be good to test this PR on Windows.

[arp242]

On Windows it returns a path like C:\Users\IEUser\AppData\Local\Temp\VIC99BD.tmp; this patch seems to work well. Thanks! 👍

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7904a27). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1601   +/-   ##
=========================================
  Coverage          ?   14.78%           
=========================================
  Files             ?       53           
  Lines             ?     4152           
  Branches          ?        0           
=========================================
  Hits              ?      614           
  Misses            ?     3538           
  Partials          ?        0

Flag	Coverage Δ
#nvim	5.05% <0%> (?)
#vim74	14.71% <100%> (?)
#vim80	14.47% <100%> (?)

Impacted Files	Coverage Δ
autoload/go/fmt.vim	63.63% <100%> (ø)

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