Support for style parameter

[TorstenC]

Thank you for the great work on this library!

People want to display all badges in the same style, but styles do not work for deps.rs-status.svg:

(?style=plastic, not plastic)
(?style=social, not social)
(?style=flat, for reference)

Other badges support the style parameter:

(?style=plastic)
(?style=social)
(?style=flat-square)

I would like to display all my badges in plastic-style.