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_gosimple_command_callback.vader

@@ -8,3 +8,12 @@ After:
 Execute(The default gosimple command should be correct):
   AssertLinter 'gosimple',
   \ ale#path#CdString(expand('%:p:h')) . ' gosimple .'
+
+Execute(The gosimple command should support Go environment variables):
+  let b:ale_go_go111module = 'on'
+
+  AssertLinter 'gosimple',
+  \ ale#path#CdString(expand('%:p:h')) . ' '
+  \ . ale#Env('GO111MODULE', 'on') . 'gosimple .'
+
+  unlet! b:ale_go_go111module