Href parsing in SVG use directive

[DylanPaulusSEL]

(https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/xlink:href)
xlink:href is being deprecated for href

Version

2.3.0

Test Case

http://jsfiddle.net/uybeqc5r/

Steps to reproduce

Use loadSVGFromUrl/loadSVGFromString with an SVG <use /> tag that contains href instead of xlink:href. (<use href="#myId" />)

Expected Behavior

Should parse the SVG into the fabric canvas.

Actual Behavior

Throws a null reference error on xlink = el.getAttribute('xlink:href').substr(1) when using href.

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Willing to make a PR for this if everything seems fine. Thinking of adding checks for either xlink:href and href. (eg. xlink = (el.getAttribute('xlink:href') || el.getAttribute('href')).substr(1)

[asturur]

it make sense to support href on top of xlink:href to do not immediately being completely incompatible with svg 2.

But we still focus on svg 1.1 ( with 1.2 addons where possible ) compatibility, so is not deprecated for us.

Most of svg2 features are really out of reach for us.

[asturur]

merged