Add the collapse option to Panel component. Closes issue #21

[themcaffee]

Should close issue #21.

I have:

• Added the new parameter to the documentation under the accordion section.
• Added the collapse option to Panel
• Changed the CSS class accordion-toggle to toggle-collapse to better reflect the purpose of the class

The collapse option allows for a panel to be collapsible without needing to put it in an accordion. If the panel is in an accordion, the collapse option will override the behavior of the accordion collapse.