Add config option to pass flags

[jbgcarnes]

Very useful extension for some of my work. It would be nice if you could customize in settings the flags you pass to trivy when you call it.

While there is the config file, configs for me are not useful for others and shouldn't be in the config file, so currently there is no good way of setting it in the current extension.

There are a number of extensions which have options like this (the Go extension) that should be easy to lift and shift