Adding `account` syntax

<!-- account: twitter, about_hiroppy -->
<!-- account: github, hiroppy -->
<!-- account: facebook, yuta.hiroto0429 -->
<!-- account: linkedin, hiroppy -->

<!-- if you want to use grid -->

<!-- block-start: grid -->
<!-- account: twitter, about_hiroppy -->
<!-- account: github, hiroppy -->
<!-- account: facebook, yuta.hiroto0429 -->
<!-- account: linkedin, hiroppy -->
<!-- block-end -->

You can set these properties using css variables.

:root {
  --account-icon-size: 3rem;
  --account-icon-color: #545454;
}

and attaches these classes

.account {};

.account-twitter,
.account-github,
.account-facebook,
.account-linkedin {}

The Slide: https://hiroppy.github.io/fusuma/intro/#slide-14