README.md

Add rainbow effect to quoted string

Configuration

Add rainbowString.extensions to settings.json

"rainbowString": {
    "extensions": [
        ".cs",
        ".csx",
        ".fs",
        ".fsx",
        ".js",
        ".jsx",
        ".ts",
        ".tsx",
        ".csproj",
        ".fsproj"
    ]
}