Made falling back to substitute configurable & added count to seek motion

Added functionality for seeking with counts (e.g. 2sco will go to the 2nd occurrence of 'co' in the same line) while sacrificing the ability to use '<count>s' for substitution (I never use it). Also added a configuration option (`g:seek_subst_disable`) which defaults to `0`. The default leaves the behaviour of seek.vim unchanged.