bump version to 2.1.0

minor code improvement

bump version to 2.0.0

bugfix: use getter to access window-vector

related with #4 and #5: there is a crash from time to time when frame-local and
winum-assign-func reserves numbers.

return empty string instead of nil when window does not exist

protects against potential errors being thrown and probably fixes #3 (although
understanding the root cause is still necessary).

Add type and group to winum-auto-setup-mode-line