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_staticcheck_command_callback.vader

@@ -24,3 +24,20 @@ Execute(The staticcheck `lint_package` option should use the correct command):
 
   AssertLinter 'staticcheck',
   \ ale#path#CdString(expand('%:p:h')) . 'staticcheck .',
+
+Execute(The staticcheck callback should use the `GO111MODULE` option if set):
+  let b:ale_go_go111module = 'off'
+
+  AssertLinter 'staticcheck',
+  \ ale#path#CdString(expand('%:p:h'))
+  \   . ale#Env('GO111MODULE', 'off')
+  \   . 'staticcheck '
+  \   . ale#Escape(expand('%' . ':t'))
+
+  " Test with lint_package option set
+  let b:ale_go_staticcheck_lint_package = 1
+  AssertLinter 'staticcheck',
+  \ ale#path#CdString(expand('%:p:h'))
+  \   . ale#Env('GO111MODULE', 'off')
+  \   . 'staticcheck .'
+  unlet! b:ale_go_go111module