tab completion enhancements

[stevesims]

currently tab completion will, on finding a match, fill in the remainder of the match and then place the cursor at the end of the line

ideally when tab-completing the cursor should be left at the end point of the text that has been inserted

additionally in the event of multiple matches it would be nice to be able to cycle through them by repeatedly pressing tab. technically, this would mean keeping track of where the insertion point as at the end of the last tab-complete to allow for the previous version to be removed