Configurable blame gutter color/image

[AndreasBackx]

Currently the blue SVG used in the sidebar in the blame is not configurable. It would be neat to have the color or the image itself be configurable. Currently you said it's using SVG because of VS Code API limitations. Is there perhaps already an issue on VS Code that you know of related to this limitation?

Related: #360.

[eamodio]

This is blocked by: microsoft/vscode#103241