Added support for template converters

[nmetulev]

PR Type

• Feature

Description of the changes

This PR adds support for converters in our templates. Ex:

<mgt-agenda id="myDay">
    <template data-type="event">
        <div>
            <div>{{ event.subject }}</div> <!-- this replaces event.subject with value -->
            <div>{{{ myConverter(event.subject }}}</div> <!-- this evaluates the expression -->
        </div>
    </template>
</mgt-agenda>

<script>
    let agenda = document.getElementById('myDay');
    agenda.templateConverters.myConverter = str => str.toUpperCase();
</script>

PR checklist

• Added tests and all passed
• All public classes and methods have been documented
• Added appropriate documentation
• License header has been added to all new source files
• Contains NO breaking changes

Other information