Make separator in RelabelConfig configurable

[martin-helmich]

Hi! @martin-helmich After I checked the source code, I found there is only the default separator as " ", I think it would be more configurable if we could choose whatever separator we need.

// RelabelConfig is a struct describing a single re-labeling configuration for taking
// over label values from an access log line into a Prometheus metric
type RelabelConfig struct {
	TargetLabel string              `hcl:",key" yaml:"target_label"`
	SourceValue string              `hcl:"from" yaml:"from"`
	Whitelist   []string            `hcl:"whitelist"`
	Matches     []RelabelValueMatch `hcl:"match"`
	Split       int                 `hcl:"split"`

	WhitelistExists bool
	WhitelistMap    map[string]interface{}
}

Originally posted by @runzhliu in #132 (comment)