Add allowed_chars parameter to slugify Liquid filter

It would be nice to add new `allowed_chars` parameter to slugify filter.

Example:

  * `{{ "C++" | slugify }}` => `c`
  * `{{ "C++" | slugify: "+" }}` => `c++`
  * `{{ "The _config.yml file" | slugify }}` => `the-config-yml-file`
  * `{{ "The _config.yml file" | slugify: '_.' }}` => `the-_config.yml-file`


I want to use this parameter when I enumerate tags of a post, because both `{{ "C++" | slugify }}` and `{ "C#" | slugify }}` are converted to `c`.

```
{% for tag in page.tags %}
<a href="/tags/{{ tag | slugify:"+#" | uri_escape }}/">{{tag | xml_escape }}</a>
{% endfor %}
```