Add support for inline VAST XML

[torbenbrodt]

Is your feature request related to a problem? Please describe.
Many SSPs like Xandr/AppNexus return the VAST XML as string in the bidresponse.
I want to use this video player to play such videos.
Unfortunately this video player only supports URLs, via the config.src attribute.

Describe the solution you'd like
The library can handle VAST XML as string when using URL Blobs.
If we add an example more people will know about it.

Describe alternatives you've considered
As an alternative we can provide a custom options.urlhandler which does not require XHR and parses the string directly.

Additional context
Bidresponse of Xandr

Example for Inline VAST

<?xml version="1.0" encoding="UTF-8"?>
<VAST xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="vast.xsd" version="3.0">
    <Ad>
        <InLine>
            <Creatives>
                <Creative sequence="1">
                    <Linear>
                        <Duration>00:01:00</Duration>
                        <VideoClicks>
                            <ClickThrough id="GDFP"><![CDATA[https://iabtechlab.com]]></ClickThrough>
                        </VideoClicks>
                        <MediaFiles>
                            <MediaFile id="GDFP" delivery="progressive" type="video/mp4"><![CDATA[https://aba.gumgum.com/13861/8/big_buck_bunny_640x360.mp4]]></MediaFile>
                        </MediaFiles>
                    </Linear>
                </Creative>
            </Creatives>
        </InLine>
    </Ad>
</VAST>

[edrpls]

@torbenbrodt Just wanted to confirm that #100 fixes this issue before closing it.

[edrpls]

Closing because of #100. This might get first-class support in the future.

Thanks, @torbenbrodt, for your contribution!