Add code coverage PR comments

[leizor]

This PR adds a step to the Drone CI configuration that adds a PR comment w/ code coverage information. An example of this comment is here.

[grafanabot]

Go coverage report:

Source	Func	%
github.com/grafana/influx2cortex/pkg/influx/api.go:33:	RegisterFlags	0.0%
github.com/grafana/influx2cortex/pkg/influx/api.go:44:	Register	0.0%
github.com/grafana/influx2cortex/pkg/influx/api.go:48:	NewAPI	0.0%
github.com/grafana/influx2cortex/pkg/influx/api.go:68:	handleSeriesPush	0.0%
github.com/grafana/influx2cortex/pkg/influx/errors.go:36:	errorHandler	0.0%
github.com/grafana/influx2cortex/pkg/influx/errors.go:74:	isNetworkTimeout	0.0%
github.com/grafana/influx2cortex/pkg/influx/errors.go:83:	isGRPCTimeout	0.0%
github.com/grafana/influx2cortex/pkg/influx/parser.go:21:	parseInfluxLineReader	88.2%
github.com/grafana/influx2cortex/pkg/influx/parser.go:50:	writeRequestFromInfluxPoints	85.7%
github.com/grafana/influx2cortex/pkg/influx/parser.go:70:	influxPointToTimeseries	76.7%
github.com/grafana/influx2cortex/pkg/influx/parser.go:134:	replaceInvalidChars	83.3%
github.com/grafana/influx2cortex/pkg/influx/parser.go:153:	batchReadCloser	100.0%
total:	(statements)	45.2%