enable sorting in the dotplot y-axis

[gaospecial]

The y-axis in dotplot may be KEGG pathway, GO term or something like that. Currently, the order of these terms are basically sorted by alphabeta of term names, and this has a disadvantage in visualizing them when there are many different terms. As far as I know, these terms are somehow have inner relationships. For example, two KEGG pathways may have many overlapped genes. Therefore, to order these terms in y-axis will be meaningful and may lead to a more beautiful figure.

Orders may be:

1. the term similarity;
2. whether term representing a activation or suppression in biology;
3. the overlap of term contained genes.

[gaospecial]

similar enhancement for #65