String slug format faker (#505)

* Add pattern for 'slug' format

* Add recognition of 'slug' format

src/generators/coreFormat.js

@@ -13,6 +13,7 @@ const regexps = {
   hostname: '[a-zA-Z]{1,33}\\.[a-z]{2,4}',
   ipv6: '[a-f\\d]{4}(:[a-f\\d]{4}){7}',
   uri: URI_PATTERN,
+  slug: '[a-zA-Z\\d_-]+',
 
   // types from draft-0[67] (?)
   'uri-reference': `${URI_PATTERN}${PARAM_PATTERN}`,

src/types/string.js

@@ -39,6 +39,7 @@ function generateFormat(value, invalid) {
     case 'idn-email':
     case 'idn-hostname':
     case 'json-pointer':
+    case 'slug':
     case 'uri-template':
     case 'uuid':
       return coreFormat(value.format);