Add fullheight modifier to jumbotron

[jmjuanes]

Add a modifier for the jumbotron to occupy the screen height (min-height: 100vh). This modifier will be called siimple-jumbotron--fullheight:

<div class="siimple-jumbotron siimple-jumbotron--fullheight">
    Jumbotron content
</div>

When using this modifier, the content of the jumbotron should be vertically aligned and the padding top and botton should be removed.

Tasks

• Add siimple-jumbotron--fullheight modifier.
• Update documentation.