Support assignment of attribute litterals via interpolation

a lot of third-party functionality is achieved via adding attributes to an element. Some of these attribute values can be quite large and/or can be best assigned via interpolation. It would be useful to support this for the sake of better integration with standards + third-party libraries.

As per @jdanyow's suggestion, introduciton of `.attr` binding could facilitate these use cases.

For example:

```html

```
would result in the outcome (assuming `current = 0`):
```html

Start by locating the templating engine's pluggable command syntax and the existing binding commands in this repository. Compare their behavior with the issue's aria-valuemin.attr and aria-valuemax.attr example, then verify that interpolated values are assigned as ordinary attributes and add or run coverage for that behavior if an existing test entry point is found.

Written by the indexing model from the issue text.