Break shared handlers up into their own files, and fix stylelint error handling

ale_linters/go/gofmt.vim

@@ -6,5 +6,5 @@ call ale#linter#Define('go', {
 \   'output_stream': 'stderr',
 \   'executable': 'gofmt',
 \   'command': 'gofmt -e %t',
-\   'callback': 'ale#handlers#HandleUnixFormatAsError',
+\   'callback': 'ale#handlers#unix#HandleAsError',
 \})

ale_linters/go/golint.vim

@@ -5,5 +5,5 @@ call ale#linter#Define('go', {
 \   'name': 'golint',
 \   'executable': 'golint',
 \   'command': 'golint %t',
-\   'callback': 'ale#handlers#HandleUnixFormatAsWarning',
+\   'callback': 'ale#handlers#unix#HandleAsWarning',
 \})

ale_linters/go/gosimple.vim

@@ -5,5 +5,5 @@ call ale#linter#Define('go', {
 \   'name': 'gosimple',
 \   'executable': 'gosimple',
 \   'command': 'gosimple %t',
-\   'callback': 'ale#handlers#HandleUnixFormatAsWarning',
+\   'callback': 'ale#handlers#unix#HandleAsWarning',
 \})

ale_linters/go/govet.vim

@@ -6,5 +6,5 @@ call ale#linter#Define('go', {
 \   'output_stream': 'stderr',
 \   'executable': 'go',
 \   'command': 'go vet %t',
-\   'callback': 'ale#handlers#HandleUnixFormatAsError',
+\   'callback': 'ale#handlers#unix#HandleAsError',
 \})

ale_linters/go/staticcheck.vim

@@ -5,5 +5,5 @@ call ale#linter#Define('go', {
 \   'name': 'staticcheck',
 \   'executable': 'staticcheck',
 \   'command': 'staticcheck %t',
-\   'callback': 'ale#handlers#HandleUnixFormatAsWarning',
+\   'callback': 'ale#handlers#unix#HandleAsWarning',
 \})