Add 'space_after' option

Adds a new option (`space_after`, defaulting to true, to preserve the current behavior) so that you can control if a space should be added in input/textareas after a match has been inserted.

If you set `space_after` to `false`, then no space is added when you insert a match. The completions list stays open, but you can then type a comma, period, or space (or whatever) which will hide the completions list. This preserves a more natural flow if you're using the UI completely via keyboard (type, @something, enter/tab to insert, then "!" to end a sentence for example).