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_gobuild_command_callback.vader

@@ -14,6 +14,16 @@ Execute(The default commands should be correct):
   \ ale#path#CdString(expand('%:p:h'))
   \   . 'go test -c -o /dev/null ./'
 
+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 test -c -o /dev/null ./'
+
+  unlet! b:ale_go_go111module
+
 Execute(Extra options should be supported):
   let g:ale_go_gobuild_options = '--foo-bar'