Add end_time, end_time_formatted and duration_formatted

for presenter, day and session
Parameterize session filters and filter items
Add {{ page.session.filter_tags }}

… in presenter, day, session and sessions
Adap liquid syntax to the template
Add one tag per filter_item instead of filter
Merge pull request #30 from AntoineInsa/30addFilters

Add filters and filter items in tags so they can be manipulated by javascript.
Merge pull request #29 from AntoineInsa/29addEndTimeAndDuration

Add end_time and duration