add graphical matrix formatting for score, dynamic programming, and traceback matrices

[gregcaporaso]

this could use seaborn heatmaps, e.g. where high values get darker colors.

related to #73

[gregcaporaso]

The tabular module works for this, but it seems limited in its customizability.

from tabulate import tabulate
from IPython.core.display import HTML


def format_matrix(row_headers, col_headers, data, hide_zeros=False, cell_width=3, nonzero_val=None):
    rows = []
    for h, d in zip(str(col_headers), data):
        current_row = [h]
        for e in d:
            if e == 0:
                if hide_zeros:
                    current_row.append('')
                else:
                    current_row.append(0)
            else:
                if nonzero_val is not None:
                    current_row.append(nonzero_val)
                else:
                    current_row.append(e)
        rows.append(current_row)
    return tabulate(rows, headers=row_headers, tablefmt='html')

[gregcaporaso]

Fixed using tabular for now - it works, but still leaves something to be desired.