Trim insignificant .00 on SI prefix format about d3-format

[alexsapps]

Same here. I want to set the number of decimal places rather than number of significant digits with SI prefixes, and I don't want to use formatPrefix because I still want the prefix to be dynamically computed for each number.

[mbostock]

Under the current design, we’d need a new format type to support this, akin to the difference between the none specifier and g but for s.

Or, we’d need to extend the format specifier syntax so that trimming of insignificant zeroes can be applied generically to any format type (any format type that makes sense, I suppose…).

[vasturiano]

@mbostock I like your suggestion to extend the syntax so that trimming can be applied generically to all types. Seems like the most reasonable option. PR open: #57.