Improve amp-accordion sample (#184)
Improve amp-accordion sample (#184)

* demonstrate better show/hide
* show simple with single show button

Fixes #182,#183.
ampproject/amp-by-example,camelburrito/amp-by-example,ampproject/amp-by-example,camelburrito/amp-by-example,ampproject/amp-by-example,camelburrito/amp-by-example,ampproject/amp-by-example,camelburrito/amp-by-example