add po support with proselint, writegood, msgfmt and alex

ale_linters/po/write-good.vim

@@ -0,0 +1,9 @@
+" Author: Cian Butler https://github.com/butlerx
+" Description: write-good for PO files
+
+call ale#linter#Define('po', {
+\   'name': 'write-good',
+\   'executable_callback': 'ale#handlers#writegood#GetExecutable',
+\   'command_callback': 'ale#handlers#writegood#GetCommand',
+\   'callback': 'ale#handlers#writegood#Handle',
+\})