Add support for single quoted attrs

[goring]

Now able to use single quotes as attribute values.

Div(attrs.Props{
    "data-values": `'{"quantity": 5}'`,
})

Previous output

<div data-values="'{" quantity": 5}'"></div>

New HTML output

<div data-values='{"quantity": 5}'></div>

Rendered by the browser

<div data-values="{"quantity": 5}"></div>

[chasefleming]

Looks good, thanks for adding this!