5.2.7.1 expr="..." attribute

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5.2.7.1 `expr="..."` attribute

You must supply an `expr` attribute containing an expression that is evaluated to generate the output `src` (5.2.7.3) attribute of the `<img>` tag.

For example:

    >>> import albatross
    >>> ctx = albatross.SimpleContext('.')
    >>> ctx.locals.src = 'httpicons.r.us/spam.png'
    >>> albatross.Template(ctx, '<magic>', '''
    ... <al-img expr="src" whitespace>
    ... ''').to_html(ctx)
    >>> ctx.flush_content()
    <img src="httpicons.r.us/spam.png" />