Proposed new filters

Added new filter call to `history-purchases.php`, in order to allow overriding its content without having to replace the whole template. Ref. https://trello.com/c/FFldj4GA/483-currency-switcher-multi-currency-solution.

Filter description and arguments can be found in commit comment. If approved, a similar filter should be added to `shortcode-receipt.php` template.

**Note**: plugin version changed to differentiate from "plain" 2.0.4, as the Currency Switcher will still load the overridden templates for previous versions.