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/command_callback/test_gofmt_command_callback.vader

@@ -0,0 +1,19 @@
+Before:
+  call ale#assert#SetUpLinterTest('go', 'gofmt')
+  call ale#test#SetFilename('../go_files/testfile2.go')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default gofmt command should be correct):
+  AssertLinter 'gofmt',
+  \ ale#Escape('gofmt') . ' -e %t'
+
+Execute(The gofmt command should support Go environment variables):
+  let b:ale_go_go111module = 'on'
+
+  AssertLinter 'gofmt',
+  \ ale#Env('GO111MODULE', 'on')
+  \ . ale#Escape('gofmt') . ' -e %t'
+
+  unlet! b:ale_go_go111module