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/gobuild.vim

@@ -11,6 +11,7 @@ function! ale_linters#go#gobuild#GetCommand(buffer) abort
 
     " Run go test in local directory with relative path
     return ale#path#BufferCdString(a:buffer)
+    \   . ale#go#EnvString(a:buffer)
     \   . ale#Var(a:buffer, 'go_go_executable') . ' test'
     \   . (!empty(l:options) ? ' ' . l:options : '')
     \   . ' -c -o /dev/null ./'