This patch add expand-abbrev-maybe. A conditional keybinding inspired
from YASnippet.

When bound to something like "TAB", it will expand abbrev if there's
some at point, it will do original binding(e.g. indent) if there's no
abbrev.

This is helpful for somebody doesn't like aggressive expand strategy in
abbrev-mode, but still doesn't want to give a exclusive keybinding for
expand-abbrev.