Transform for common aria attributes

[jamesmartin]

Addresses #28.

This branch adds a new transform that, when enabled, adds the following accessibility attributes to the SVG:

• role="img"
• aria-labelledby="title desc" (only adds 'title' and 'desc' if elements exist in the document)

By default, this transformation is not active. It can be used as follows:

inline_svg("some_document.svg", title: "some title", desc: "some description" aria: true)

This results in the following (obviously, title and description are optional):

<svg role="img" aria-labelledby="title desc"><title>Some title</title><desc>Some description</desc>Some document</svg>

/cc @samjewell for requesting this feature