Add support for arrow keys and more

[fjl5]

This pull request slightly changes the UI behavior and the API, but I still wanted to share it:

• Show suggestion as soon as any character is entered, not waiting for an "@" sign and the first character of the domain
• Use up and down arrows (cursor keys) to browse through all matches
• Don't autocomplete on focus leave, only on tab, right arrow, click
• Only handle right arrow as autocomplete at end of input. This enables users to edit the text left of the "@" character without accidentally triggering autocomplete
• Allow user to replace the list of domains, not just extend it (API change)
• Make the separator character configurable; default is of course @

[yongzhenlow]

@fjl5 Thanks for your input, I'll have a look :)