Added subcomponents

[ghost]

Hi there, i've extended the component code to allow for defining parts of the component template and then replacing them from within the master template akin to the Django's block/extend
Sample component template

    <div class="panel panel-default">
        <div class="panel-heading">
            <rv-bs-panel-heading>
                <h3 class="panel-title" rv-text="item.title"></h3>
            </rv-bs-panel-heading>
        </div>
        <div class="panel-body">
            <rv-bs-panel-content/>
        </div>
    </div>

Sample master template

    <rv-bs-panel rv-each-i="arr" item="i">
        <rv-bs-panel-content>
            <b rv-text="item.content.param1"></b>: <i rv-text="item.content.param2"></i>
        </rv-bs-panel-content>
    </rv-bs-panel>

    <rv-bs-panel rv-each-i="arr" item="i">
        <rv-bs-panel-heading>
            <h1 rv-text="item.title"></h1>
        </rv-bs-panel-heading>
        <rv-bs-panel-content>
            <b>{item.content.param1}</b> - <i>{item.content.param2}</i>
        </rv-bs-panel-content>
    </rv-bs-panel>

And sample data

{
  "arr": [
    {
      "title": "Test header",
      "content": {
        "param1": "one",
        "param2": "two"
      }
    }, {
      "title": "Testowy naglowek",
      "content": {
        "param1": "jeden",
        "param2": "dwa"
      }
    }
  ]
}

[stalniy]

@sszelag in order to see this in lib you need to add tests. Also I suggest you to take a look at what I've described in #553