Allow custom markers with values

[justenstall]

Allow custom markers to have a value like kubebuilder markers.

type Config struct {
   // +kubebuilder:default=25
   MaxOpenConns int `json:"maxOpenConns,omitempty"`
}

Motivation is to define an env custom marker to generate an extra column with the equivalent environment variable name.

type Config struct {
   // +env="MAX_OPEN_CONNS"
   MaxOpenConns int `json:"maxOpenConns,omitempty"`
}

I tried implementing this using the existing processor.customMarkers config and modified templates like the examples, but it seems like the markers are not matched if they are followed by an equals sign.