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_bingo_command_callback.vader

@@ -9,6 +9,7 @@ After:
   endif
 
   unlet! b:ale_completion_enabled
+  unlet! b:ale_go_go111module
 
   call ale#assert#TearDownLinterTest()
 
@@ -34,6 +35,14 @@ Execute(should set bingo options):
   AssertLinter 'bingo',
   \ ale#Escape('bingo') . ' --mode stdio --trace'
 
+Execute(should support Go environment variables):
+  call ale#test#SetFilename('go_paths/go1/prj1/file.go')
+  let b:ale_go_go111module = 'on'
+
+  AssertLinter 'bingo',
+  \ ale#Env('GO111MODULE', 'on') . ale#Escape('bingo') . ' --mode stdio'
+
+
 Execute(Should return directory for 'go.mod' if found in parent directory):
   call ale#test#SetFilename('../go_files/test.go')
 
@@ -44,3 +53,20 @@ Execute(Should return nearest directory with '.git' if found in parent directory
   call mkdir(g:dir . '/.git')
 
   AssertLSPProject g:dir
+
+Execute(Should ignore 'go.mod' and return '.git' dir if modules off):
+  call ale#test#SetFilename('../go_files/test.go')
+
+  let b:ale_go_go111module = 'off'
+  let b:parent_dir = ale#path#Simplify(g:dir . '/..')
+  let b:git_dir = b:parent_dir . '/.git'
+
+  if !isdirectory(b:git_dir)
+    call mkdir(b:git_dir)
+  endif
+
+  AssertLSPProject b:parent_dir
+
+  call delete(b:git_dir, 'd')
+  unlet! b:parent_dir
+  unlet! b:git_dir