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`

test/command_callback/test_golangserver_command_callback.vader

@@ -14,6 +14,7 @@ After:
   Restore
 
   unlet! b:ale_completion_enabled
+  unlet! b:ale_go_go111module
   unlet! g:sep
 
   call ale#assert#TearDownLinterTest()
@@ -52,6 +53,12 @@ Execute(should ignore go-langserver -gocodecompletion option):
 
   AssertLinter 'go-langserver', ale#Escape('go-langserver') . ' -trace'
 
+Execute(should support Go environment variables):
+  let b:ale_go_go111module = 'on'
+
+  AssertLinter 'go-langserver',
+  \ ale#Env('GO111MODULE', 'on') . ale#Escape('go-langserver')
+
 Execute(should set go-langserver for go app1):
   call ale#test#SetFilename('go_paths/go1/prj1/file.go')