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`

test/fixers/test_gofmt_fixer_callback.vader

@@ -5,6 +5,7 @@ Before:
   " Use an invalid global executable, so we don't match it.
   let g:ale_go_gofmt_executable = 'xxxinvalid'
   let g:ale_go_gofmt_options = ''
+  let g:ale_go_go111module = ''
 
   call ale#test#SetDirectory('/testplugin/test/fixers')
 
@@ -38,3 +39,16 @@ Execute(The gofmt callback should include custom gofmt options):
   \     . ' %t',
   \ },
   \ ale#fixers#gofmt#Fix(bufnr(''))
+
+Execute(The gofmt callback should support Go environment variables):
+  let g:ale_go_go111module = 'off'
+  call ale#test#SetFilename('../go_files/testfile.go')
+
+  AssertEqual
+  \ {
+  \   'read_temporary_file': 1,
+  \   'command': ale#Env('GO111MODULE', 'off')
+  \     . ale#Escape('xxxinvalid') . ' -l -w'
+  \     . ' %t',
+  \ },
+  \ ale#fixers#gofmt#Fix(bufnr(''))