Add Support to html markup in I18n options.

This PR fixes #764.

Now, it is possible to have keys in collections which contains html markup, and these keys are going to be escaped.

Example:

The following input:

```erb
<%= f.input :gender, collection: [:female, :male] %>
```

and the following keys:

```YAML
user:
  gender:
    female_html: '<strong>Female</strong>'
    male_html: '<strong>Male</strong>'
```

Will now result in this html, properly escaping the html.

```HTML
<div class="input radio_buttons required user_gender">
  <label class="radio_buttons required">
    <abbr title="required">*</abbr> Gender
  </label>
  <span class="radio radio">
    <label for="user_gender_male" name="user[gender]">
      <input aria-required="true" class="radio_buttons required" id="user_gender_male" name="user[gender]" required="required" type="radio" value="male" />
      <strong>Male</strong>
    </label>
  </span>
  <span class="radio radio">
    <label for="user_gender_female" name="user[gender]">
      <input aria-required="true" class="radio_buttons required" id="user_gender_female" name="user[gender]" required="required" type="radio" value="female" />
      <strong>Female</strong>
    </label>
  </span>
</div>
```

Yes, I thought the same but I could not find a better way.