Use tabular input data.

[mbostock]

TODO

• How are nodes and parents identified? id and parentId?
• Can we specify internal nodes implicitly (e.g., a CSV of leaves)?
• How hard is it to migrate the old tree structure to a flat table?
• Can internal nodes have value? (Related Should internal nodes have (self) values? #9 Added ability to specify additional size for hierarchical parents d3#2379.)

[emeeks]

A vote for support for the last point, since we've seen how effective flame graphs/charts can be and they require the ability to set the value of an non-leaf node for the existing partition layout.

[mbostock]

Folding into #14.