Formatting options for StringElements

[alecperkins]

The StringElement should allow for specifying formatting, since right now it just dumps the raw value. Something like [default content]{var_name: %.2f}.

[alecperkins]

For consistency with the NumberElement display precision and to avoid the rather opaque % formatting, a simpler way would be [default content]{var_name: %0.00} or even [default content]{var_name: 0.00}. Having the % is a nice differentiator, but may be confusing for those who think it means percent or modulo, or for those who expect it to be conventional sprintf-type formatting.

But then there are dates, for which a formatting string would be useful, eg [text content]{var_name: %b %-d, %Y}. (That also raises the question of date input.)