Add new 'groupTpl' feature

[davidpede]

What does it do?

Adds a new optional tpl parameter that wraps all tags with their relevant group details.

Believe I've kept this backward compatible. Group placeholders are available using group. prefix, tags are available under [[+tags]]. rowTpl and outTpl are unaffected.

Use like:

[[TaggerGetTags?
    &groups=`2,3,4,5`
    &groupTpl=`@INLINE <optgroup label="[[+group.name]]">[[+tags]]</optgroup>`
    &rowTpl=`@INLINE <option value="[[+uri]]">[[+tag]]</option>`
]]

Why is it needed?

Adds support for things like optgroup where you need the group placeholders and all the tags the group contains in one tpl.