Create subfunctions: count_offtargets(), filter_offtargets. Unexport add_offtargets and add_ontargets

Add argument "offtargetmethod" to find_spacers, filter_offtargets, add_offtargets, add_genome_counts, add_target_counts, count_spacer_matches,

find_spacers: add argument subtract_targets to allow to subtract target (mis)matches from offf-target counts.