Add port to the list of options

PORT env variable is used by gin, but adding it to the list of config options will generate a flag for it and provide value validation

README.md

@@ -195,6 +195,10 @@ HTTP port to listen on. Example:
 
     PORT=8000
 
+This option can also be set using `-port` flag. Example:
+
+    $ unsee -port 8000
+
 Default is `8080`.
 
 #### SENTRY_DSN

config/config.go

@@ -30,6 +30,7 @@ type configEnvs struct {
 	Debug               bool               `envconfig:"DEBUG" default:"false" help:"Enable debug mode"`
 	FilterDefault       string             `envconfig:"FILTER_DEFAULT" help:"Default filter string"`
 	JiraRegexp          spaceSeparatedList `envconfig:"JIRA_REGEX" help:"List of JIRA regex rules"`
+	Port                int                `envconfig:"PORT" default:"8080" help:"HTTP port to listen on"`
 	SentryDSN           string             `envconfig:"SENTRY_DSN" help:"Sentry DSN for Go exceptions"`
 	SentryPublicDSN     string             `envconfig:"SENTRY_PUBLIC_DSN" help:"Sentry DSN for javascript exceptions"`
 	StripLabels         spaceSeparatedList `envconfig:"STRIP_LABELS" help:"List of labels to ignore"`