Support pasting of values from Excel

[nowthis]

Instead of insisting on the SankeyMATIC format for specifying flows, allow people to just copy some cells from a spreadsheet and paste them in & get the results they expect.

I believe the right way to support this would be to expect three columns of pasted data.

• Figure out which column has the numeric values. Could be column 1 or 2 or 3
• Once we know which two columns are not the values, presume the first column of the two is the Source & the second is the Target.
• Rearrange the values into the SankeyMATIC syntax
• Paste the result into the data-editing field.
• Handle sensing & skipping a header row, maybe.