Allow to change next/prev mappings

[aostruszka]

User can override/extend next/previous mappings by defining dictionary 'g:unimpaired_np_maps' e.g.:

let g:unimpaired_np_maps = { 't': 'tab' }

will change [t,]t,[T,]T mappings to work for tabs (_tab_prev,...) instead of default mapping for tags (_t_prev,...).