Sequence sort for plot_sequence_heatmap

[snystrom]

using a lexical sort for plot_sequence_heatmap could be one way to improve the visualization. However, a naive lexical sort won't capture the "core" of a motif correctly, and will make consistency in the "shoulders" pop. A few ideas are to use lexical sorting by providing a positional index on which to sort.

Other thoughts:

• weight alphabet differently? Instead of lexical, weight by nt frequency at position?