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/golint.vim

@@ -7,7 +7,7 @@ call ale#Set('go_golint_options', '')
 function! ale_linters#go#golint#GetCommand(buffer) abort
     let l:options = ale#Var(a:buffer, 'go_golint_options')
 
-    return '%e'
+    return ale#go#EnvString(a:buffer) . '%e'
     \   . (!empty(l:options) ? ' ' . l:options : '')
     \   . ' %t'
 endfunction