Support pretty-printing and whitespace management

[davepeck]

I need to think more carefully about whitespace handling.

You can’t just trim all the strings, since whitespace is (to a degree) significant in HTML.

At the same time, precise whitespace preservation from t-string literal to final output is clearly not a goal: some of the whitespace in the HTML we process will be incidental, as a side effect of code formatting in Python itself.

I propose adding a render() method to the Node hierarchy; str() will invoke it directly. We should support a “compact” (all whitespace removed except significant whitespace) and a “pretty” (multiline and indented, for debugging if for nothing else) output mode. Probably we need to support an "exact" mode as well, since <pre> tags, etc. should keep spacing unaltered.