Provide option to disable amp-boilerplate CSS

[pradeep910]

In some cases where CLS is not the issue, we won't need following CSS.

<style amp-boilerplate>
@-webkit-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-moz-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-ms-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-o-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}
.site-content{-webkit-animation:-amp-start 1s steps(1,end) 0s 1 normal both;-moz-animation:-amp-start 1s steps(1,end) 0s 1 normal both;-ms-animation:-amp-start 1s steps(1,end) 0s 1 normal both;animation:-amp-start 1s steps(1,end) 0s 1 normal both}
@media (min-width: 768px) {.site-content{-webkit-animation:-amp-start 0.5s steps(1,end) 0s 1 normal both;-moz-animation:-amp-start 0.5s steps(1,end) 0s 1 normal both;-ms-animation:-amp-start 0.5s steps(1,end) 0s 1 normal both;animation:-amp-start 0.5s steps(1,end) 0s 1 normal both} }
</style>
<noscript><style amp-boilerplate>.site-content{-webkit-animation: none;-moz-animation: none;-ms-animation: none;animation: none}</style></noscript>

• [x] Provide option in the plugin settings to disable amp-boilerplate CSS.

[Souptik2001]

Covered in #15.