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`

ale_linters/go/gosimple.vim

@@ -2,7 +2,8 @@
 " Description: gosimple for Go files
 
 function! ale_linters#go#gosimple#GetCommand(buffer) abort
-    return ale#path#BufferCdString(a:buffer) . ' gosimple .'
+    return ale#path#BufferCdString(a:buffer) . ' '
+    \   . ale#go#EnvString(a:buffer) . 'gosimple .'
 endfunction
 
 call ale#linter#Define('go', {