Add progress_bar helper

This stems from the amazing work of @buren (https://github.com/buren/bh/commit/a8ad37c70da8c84fad66d00481426e180a3494a5 and https://github.com/buren/bh/commit/fb75487597fe92490dfa44e16315f7b453882050) applying the comments listed at https://github.com/Fullscreen/bh/pull/15

The syntax for progress bar would be the following. Thumbs up @buren  ?

```rhtml
Basic

<%= progress_bar %>

<%= progress_bar percentage: 60 %>

With label

<%= progress_bar percentage: 60, label: true %>

<%= progress_bar percentage: 60, label: "two thirds completed" %>

Low percentages

(nothing to do)


<%= progress_bar percentage: 0, label: true %>


<%= progress_bar percentage: 2, label: true %>

Context

<%= progress_bar percentage: 60, context: :success %>

Striped

<%= progress_bar percentage: 60, striped: true %>

Animated

<%= progress_bar percentage: 60, animated: true %>

Stacked

<%= progress_bar [{percentage: 30, context: :success}, {percentage: 40, context: :warning}] %>
```