Add *.po file headers

[StVolodymyr]

Extend hardcoded headers for *.po file

grunt.initConfig({
  nggettext_extract: {
    pot: {
      options:{
        headers: {
          'Language': 'en_US'
        }
      },
      files: {
        'po/template.pot': ['src/views/*.html']
      }
    },
  },
})