Support filling arbitrary attributes with generated strings.

[cburschka]

Right now, the string-loader supports setting .text() (via .string[data-string]) and .html() (via .string-html[data-string-html]) and .attr('title') (via .string-title[data-string-title]).

For #94, I want to add a localized placeholder attribute to the search bar.

Instead of adding a new special .string-placeholder class here, it would be better to normalize this as follows:

• All .string elements get the same treatment.
• If there is a data-string attribute, the corresponding string becomes their inner text.
• For every data-string-(.*) attribute, the corresponding string becomes their $1 attribute.

.string-html is still a special case, but the data attribute is renamed to data-string to keep the attribute namespace compact.

[cburschka]

Actually the string-html thing is only used in one place. If we can get rid of that, it'd help.

(This is because the above patch would make it impossible to add HTML and attributes to the same element.)