Use stdin/stdout to communicate with gofmt

[andreypopp]

Problem: I use docker based workflow where all devtools including gofmt are inside a container. The vim is running on host system though and runs gofmt via a wrapper script which does docker exec gofmt. Because currently fixer uses temp files gofmt cannot see those files produced on the host system.

Solution: use stdin/stdout to communicate with gofmt

I might be missing something why it's not using stdin/stdout currently.

cc @aliou @eliath as contributors to this fixer

[stale]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests.
If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.

[andreypopp]

@w0rp hi, can you take a look 👀?

[stale]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests.
If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.

[andreypopp]

Not stale!

[hsanson]

Simple and works fine with my limited testing on some small go projects. Probably gofmt did not support stdin/stdout before and that is why ALE did not use it this way.