Support mapping rules of repositories using configuration files

[marefr]

Given #11 is implemented it would be nice to have support for configuration of ignore and mapping rules of repositories using configuration files instead as how it works today, in code:

devtools/pkg/githubstats/githubstats.go

Lines 58 to 113 in 9b8d291

	skipRepos = []string{
	"grafana/ansible-grafana",
	"grafana/appdynamics-grafana-datasource",
	"grafana/app-plugin-example",
	"grafana/azure-marketplace",
	"grafana/cortex",
	"grafana/datasource-plugin-generic",
	"grafana/datasource-plugin-genericdatasource",
	"grafana/devtools",
	"grafana/dockerana",
	"grafana/docs-base",
	"grafana/example-app",
	"grafana/fake-data-gen",
	"grafana/gemini-scrollbar",
	"grafana/github-repo-metrics",
	"grafana/github-to-es",
	"grafana/globalconf",
	"grafana/grafana-api-golang-client",
	"grafana/grafana-api-nodejs-client",
	"grafana/grafana-cli",
	"grafana/grafana-docker-dev-env",
	"grafana/grafana.org",
	"grafana/grafana-packer",
	"grafana/grafana_plugin_model",
	"grafana/grafana-plugin-model",
	"grafana/grafana-plugins",
	"grafana/grafana-tools",
	"grafana/grafana-wixtoolset",
	"grafana/gridstack.js",
	"grafana/heatmap-panel",
	"grafana/homebrew-core",
	"grafana/homebrew-grafana",
	"grafana/hosted-metrics-sender-example",
	"grafana/mt-qa",
	"grafana/plugin_model",
	"grafana/puppet-grafana",
	"grafana/react-grid-layout",
	"grafana/snap_k8s",
	"grafana/snap-plugin-collector-cadvisor",
	"grafana/snap-plugin-collector-gitstats",
	"grafana/snap-plugin-collector-kubestate",
	"grafana/starter-panel",
	"grafana/synthesize",
	"grafana/usage-stats-handler",
	"grafana/vertica-datasource",
	}
	mapRepos = map[string]string{
	"grafana/": "grafana/grafana",
	"grafana/grafana-docker": "grafana/grafana",
	"grafana/grafana-build-container": "grafana/grafana",
	"grafana/azure-kusto-datasource": "grafana/azure-data-explorer-datasource",
	"grafana/datasource-plugin-influxdb-08": "grafana/influxdb-08-datasource",
	"grafana/datasource-plugin-kairosdb": "grafana/kairosdb-datasource",
	"grafana/panel-plugin-piechart": "grafana/piechart-panel",
	}

Wanted features:

• Define patterns for ignoring repositories by name.
• Define patterns/rules for mapping a repository name to some other repository name. This is needed to support "merging of repositories" when for example a repository have been archived and incorporated in some other repository,