bug#15485: add-abbrev: don't use hard-coded forward-word

[Andreas Röhler]

add-abbrev uses forward-word to catch the expansion, which isn't good for 
programming code.

See

http://stackoverflow.com/questions/19073674/how-can-i-add-my-personal-abbreviation-to-emmet-mode-in-emacs/19079683#19079683

Suggest to use forward-symbol instead, or still better to make if customizable:

(funcall add-abbrev-move-function