Implement skipWord option

Fixes #16

README.md

@@ -120,6 +120,26 @@ test('Capitalize words, handling shorthand ordinals (1st, 2nd, 3rd) correctly',
 })
 ```
 
+and thanks to a suggestion from [@Songyu-Wang](https://github.com/grncdr/js-capitalize/issues/16), capitalize supports skipping words that should never be capitalized (e.g.  articles and coordinating conjunctions in English).
+
+```javascript
+test('Title-case words, by providing a skipWord regex', function (t) {
+  t.plan(1)
+  var opts = { skipWord: /^(a|the|an|and|or|but|in|on|of|it)$/ }
+  t.equal(
+    capitalize.words('the story of an unlikely feature and the way it was implemented', opts),
+    'The Story of an Unlikely Feature and the Way it Was Implemented'
+  )
+})
+
+test('Title-case words, by providing a skipWord callback', function (t) {
+  t.plan(1)
+  t.equal(
+    capitalize.words('this is nice eh?', { skipWord: (word) => word.length < 3 }),
+    'This is Nice eh?'
+  )
+})
+```
 
 ## Install
 

index.js

@@ -20,6 +20,7 @@ capitalize.words = function (string, opts) {
   var nonWord = /[^0-9a-zA-Z\u00C0-\u017F\u0400-\u04FF]+|$/g
   var match
   var out = ""
+  var count = 0
 
   while (match = nonWord.exec(string)) {
     var sep = match[0]
@@ -34,8 +35,14 @@ capitalize.words = function (string, opts) {
       out += word[0]
       word = word.substring(1)
     }
-    out += capitalize(word, opts) + sep
+    if (typeof opts.skipWord === 'function' && opts.skipWord(word, count)) {
+      out += word
+    } else {
+      out += capitalize(word, opts)
+    }
+    out += sep
     startOfWord = nonWord.lastIndex
+    count++
     if (startOfWord == string.length) {
       break
     }
@@ -51,6 +58,12 @@ function normalizeOptions(opts) {
   if (typeof opts === 'boolean') {
     return { preserve: opts }
   }
+  if (opts.skipWord instanceof RegExp) {
+    const rgx = opts.skipWord
+    opts.skipWord = function (word, position) {
+      return position > 0 && rgx.test(word)
+    }
+  }
   return opts || {}
 }