New function for concatonating SVG files together as <symbol>s

- Create a new \DOMDocument
- Set documentElement as SVG with correct namespace, etc
- Loop through the $svgs array, reading the contents
- Use RegExps on the resulting string to change tagName & minify
- Append the resulting XML (how it's treated) to the SVG
- Save or return the results
First batch of icons as <symbol>s

Need to update usage everywhere to make use of these.
Use as <svg ><use xlink:href="#TheID" /></svg>
Need to do a file_get_contents somewhere to make use of.
Batch SVG should be hidden somehow.