Added a transform function to list helper functions

## What it does
With this litte change you can transform all values generated by the list helper function.

### Default behavior

nothing changed, because we use a simple default function:
```
  // default: do not transform the value
  transform_default = function(value) { return value; };
``` 

### Simple example - Uppercase first letter

Define a helper function (for full example see below)

```
function capitaliseFirstLetter(string)
{
    return string.charAt(0).toUpperCase() + string.slice(1);
}
```

```
    <span class="category">{{ list_categories(article.categories, {
      show_count: false,
      show_count: false,
      class: 'article-category',
      style: 'none',
      separator: '►',
      transform: capitaliseFirstLetter
    }) }}</span>
```


and use it in your templates:

### Simple example - Translations

use it with `__()` to translate things

```
    <span class="category">{{ list_categories(article.categories, {
      show_count: false,
      show_count: false,
      class: 'article-category',
      style: 'none',
      separator: '►',
      transform: __
    }) }}</span>
```

## Full example 

How to use it to translate categories, tags and more (or just give tags and categories a 'slug'). Use it like this:

in `theme/yourtheme/script/extend.js`

```
hexo.extend.helper.register('category_transform', function(helper) {
  var f = function(s) {
    return helper('category_'+s);
  }
  return f;
})

hexo.extend.helper.register('tag_transform', function(helper) {
  var f = function(s) {
    return helper('tag_'+s);
  }
  return f;
})
```

in `theme/yourtheme/languages/default.yml`

```
category_test: Test
category_it-security: IT-Sicherheit

tag_mac: Mac OS X
tag_linux: Linux
tag_javascript: Java-Script
tag_itsecurity: IT-Sicherheit
```

e.g. in a swig template:

```
    <span class="category">{{ list_categories(article.categories, {
      show_count: false,
      show_count: false,
      class: 'article-category',
      style: 'none',
      separator: '►',
      transform: category_transform(__)
    }) }}</span>

    <span class="category">{{ list_tags(article.tags, {
      show_count: false,
      class: 'article-category',
      style: 'none',
      separator: ', ',
      transform: tag_transform(__)
    }) }}</span>
```