Write a better algorithm for "flattening" spanning cells

[shawnbot]

The current one for column headers works well for this table, but I need more examples. Also, there is currently zero support for data cells with colspan or rowspan attributes.

Tests should cover:

• Spanning column headers. E.g. for this table the last three columns should be parsed as "Week Ending Mar 27", "Week Ending Mar 20", and "2015 Feb":

  

• Whitespace: <br> should be treated as space, whitespace trimmed from the left and right, and all other whitespace collapsed into single spaces.

• Spanning data cells. If a <td> has colspan="2" then the resulting row object should have that cell's text value for both of the keys of the columns it spans. Likewise, if it has rowspan="2", then the following row should "inherit" this row's value for this column.