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`

doc/ale-go.txt

@@ -29,6 +29,15 @@ g:ale_go_go_executable                                    *g:ale_go_go_options*
   The executable that will be run for the `gobuild` and `govet` linters, and
   the `gomod` fixer.
 
+g:ale_go_go111module                                     *g:ale_go_go111module*
+                                                         *b:ale_go_go111module*
+  Type: |String|
+  Default: `''`
+
+  Override the value of the `$GO111MODULE` environment variable for
+  golang tools.
+
+
 
 ===============================================================================
 bingo                                                            *ale-go-bingo*