Enable changing [ and ] to someting else.

On my (Swedish) keyboards [ means pressing two keys in windows and three on mac. That stopped me from ever trying unimpaired even though I heard so much good about it.
Now I decided to do something about it.

By setting g:unimpaired_left and g:unimpaired_right to e.g. '<' and '>' respectively, all mappings can be changed.

Default is still [ and ].

I'm not quite sure about the names for the variables. Perhaps unimpaired_prev and unimpaired_next would be better?