Refactor to improve performance w/ hoisted regex

Closes GH-39.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: bluwy

lib/handle/comment.js

@@ -7,6 +7,12 @@
 
 import {stringifyEntities} from 'stringify-entities'
 
+const htmlCommentRegex = /^>|^->|<!--|-->|--!>|<!-$/g
+
+// Declare arrays as variables so it can be cached by `stringifyEntities`
+const bogusCommentEntitySubset = ['>']
+const commentEntitySubset = ['<', '>']
+
 /**
  * Serialize a comment.
  *
@@ -27,10 +33,12 @@ export function comment(node, _1, _2, state) {
     ? '<?' +
         stringifyEntities(
           node.value,
-          Object.assign({}, state.settings.characterReferences, {subset: ['>']})
+          Object.assign({}, state.settings.characterReferences, {
+            subset: bogusCommentEntitySubset
+          })
         ) +
         '>'
-    : '<!--' + node.value.replace(/^>|^->|<!--|-->|--!>|<!-$/g, encode) + '-->'
+    : '<!--' + node.value.replace(htmlCommentRegex, encode) + '-->'
 
   /**
    * @param {string} $0
@@ -39,7 +47,7 @@ export function comment(node, _1, _2, state) {
     return stringifyEntities(
       $0,
       Object.assign({}, state.settings.characterReferences, {
-        subset: ['<', '>']
+        subset: commentEntitySubset
       })
     )
   }

lib/handle/text.js

@@ -9,6 +9,9 @@
 
 import {stringifyEntities} from 'stringify-entities'
 
+// Declare array as variable so it can be cached by `stringifyEntities`
+const textEntitySubset = ['<', '&']
+
 /**
  * Serialize a text node.
  *
@@ -32,7 +35,7 @@ export function text(node, _, parent, state) {
     : stringifyEntities(
         node.value,
         Object.assign({}, state.settings.characterReferences, {
-          subset: ['<', '&']
+          subset: textEntitySubset
         })
       )
 }