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_govet_command_callback.vader

@@ -17,3 +17,11 @@ Execute(Extra options should be supported):
 Execute(The executable should be configurable):
   let g:ale_go_go_executable = 'foobar'
   AssertLinter 'foobar', ale#path#CdString(expand('%:p:h')) . ' foobar vet  .'
+
+Execute(Go environment variables should be supported):
+  let b:ale_go_go111module = 'on'
+  AssertLinter 'go',
+  \ ale#path#CdString(expand('%:p:h')) . ' '
+  \ . ale#Env('GO111MODULE', 'on')
+  \ . 'go vet  .'
+  unlet! b:ale_go_go111module