Base64 output and class parameter

[nbrignol]

Refactored to regex, allowing me to :

• add a base64 output option
• add an optional parameter to pass a custom class (for raw or base64 output), applied to the SVG element

Usage :

./my-svg.svg?base64
./my-svg.svg?base64=customclass
./my-svg.svg?raw=customclass