Add sprintf formatting support

[rymohr]

Support standard sprintf notation with this library: http://www.diveintojavascript.com/projects/javascript-sprintf

template = "{{followers/following%0.2f}}" # round to two decimals

We'll want to build in sensible defaults too.

We'll also want to add in a shorthand notation for rounding (will be the most common operation by far):

"{{expression%0}}" => round to whole
"{{expression%n}}" => round nth decimal

Anyone that's worried about padding can use the sprintf notation.

[rymohr]

For shorthand notation we can just rely on javascript's toFixed method.