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/gofmt.vim

@@ -1,10 +1,16 @@
 " Author: neersighted <bjorn@neersighted.com>
 " Description: gofmt for Go files
 
+function! ale_linters#go#gofmt#GetCommand(buffer) abort
+    return ale#go#EnvString(a:buffer)
+    \   . '%e -e %t'
+endfunction
+
+
 call ale#linter#Define('go', {
 \   'name': 'gofmt',
 \   'output_stream': 'stderr',
 \   'executable': 'gofmt',
-\   'command': 'gofmt -e %t',
+\   'command': function('ale_linters#go#gofmt#GetCommand'),
 \   'callback': 'ale#handlers#unix#HandleAsError',
 \})