create expand_snippet method
undef tab to expand_snippet
add mikutter command to expand_snippet
move_focus after trying expand_snippet

If Tab is set to expand_snippet mikutter command,
it is not functional.
Fix it by trying expand_snippet when typed Tab
if Tab is set to expand_snippet in UserConfig.
move_focus after trying expand_snippet: for Shift+ISO_Left_Tab
get snippet list by flyweight pattern