Support $GO111MODULE with Go tooling

Allows the user to override $GO111MODULE environment variable through
ale options. This gives control over the default behavior of Go module
resolution.

Golang documentation:
https://github.com/golang/go/wiki/Modules#how-to-use-modules

Add `ale#Go#EnvString()` function to make it easy to add similar Go
environment variables in the future.

Use the new `EnvString` function in all available Go tools callbacks
& update tests

Also add test of linter command callback for `gofmt`

ale_linters/go/langserver.vim

@@ -15,8 +15,9 @@ function! ale_linters#go#langserver#GetCommand(buffer) abort
     endif
 
     let l:options = uniq(sort(l:options))
+    let l:env = ale#go#EnvString(a:buffer)
 
-    return join(extend(l:executable, l:options), ' ')
+    return l:env . join(extend(l:executable, l:options), ' ')
 endfunction
 
 call ale#linter#Define('go', {