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_golint_command_callbacks.vader

@@ -16,3 +16,11 @@ Execute(The golint options should be configurable):
   let b:ale_go_golint_options = '--foo'
 
   AssertLinter 'golint', ale#Escape('golint') . ' --foo %t'
+
+Execute(The golint command should support Go environment variables):
+  let b:ale_go_go111module = 'on'
+
+  AssertLinter 'golint',
+  \ ale#Env('GO111MODULE', 'on') . ale#Escape('golint') . ' %t'
+
+  unlet! b:ale_go_go111module