time field highlighting when picked (for always open time picker)

[olivermuc]

this is setting the 'selected' class to the HTML element picked, which is useful when the time picker stays always open and you want to provide visual feedback of the current choice made, similar to the calendar.

ideally this would be accompanied by a time setting, eg. timeAutoClose: 'never', which would avoid toggling the visibility of the picker. Currently, the workaround is to use CSS to force the time picker parent to always show. It works, but less elegantly.

this is my first pull req - feel free to implement properly, or even drop it altogether ;-)

[bevacqua]

Make the changes in the comments and please add:

• set the autoClose default value to 'auto' in src/defaults
• make sure existing uses where autoClose === true do autoClose === 'auto' instead
• add a new 'never' option for autoClose that doesn't even auto-close the time list
• documentation of changes