Adds accessible labels to range limit text fields

* Adds a `label` param to `render_range_input()` helper method 
* When a label text is provided, an accessible, hidden `'.sr-only'` `<label>` is prepended to text field HTML

```ruby
render_range_input("pub_year_tisim", :begin, "from year")
```
returns 

```html
<label class="sr-only" for="range_pub_year_tisim_begin">from year</label>
<input class="form-control range_begin" id="range_pub_year_tisim_begin" maxlength="4" name="range[pub_year_tisim][begin]" type="text" />
```

@cbeer `html = ''` line is used for concatenation when `label` param value is empty