Fails on markup declarations in Doctype declaration

[kraftner]

If your Doctype declaration contains a markup declaration is-svg fails.
I know that this package only does a quick-and-dirty check and you'd might call this an ignorable edge-case. The problem is that the default export from at least Illustrator CS4 (I know, old, but probably still not that rare) creates this...

<?xml version="1.0" encoding="utf-8"?>
<!-- Generator: Adobe Illustrator 14.0.0, SVG Export Plug-In  -->
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd" [
    <!ENTITY ns_flows "http://ns.adobe.com/Flows/1.0/">
]>
<svg version="1.1"
     xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:a="http://ns.adobe.com/AdobeSVGViewerExtensions/3.0/"
     x="0px" y="0px" width="300px" height="300px" viewBox="-62.324 -49.991 300 300" enable-background="new -62.324 -49.991 300 300"
     xml:space="preserve">
<defs>
</defs>
<rect x="0.5" y="0.5" fill="#FFFFFF" stroke="#000000" width="171.383" height="171.383"/>
</svg>

... where <!ENTITY ns_flows "http://ns.adobe.com/Flows/1.0/"> makes is-svg fail

I noticed this because this makes imagemin-svgo ignore those files.