Fix bugs introduced in previous commit

In the previous commit, when `extract()` function was removed, some bugs were introduced. I'm really sorry about that.

Alerts shortcode: in line 515 `$dismissable` shouldn't be cahnged to `$atts['dismissable']` beacuse `$dismissable` was not a reference to shortcode attribute.
Progress bar shortcode: in line 575 `$percent` should be now `$atts['percent']`.