Prometheus-es-exporter

Fixes:
- [[73](https://github.com/braedon/prometheus-es-exporter/pull/73)] Fix nodes stats parsing for AWS managed Elasticsearch clusters

Enhancements:
- [[32](https://github.com/braedon/prometheus-es-exporter/issues/32)] Add helm chart (thanks alexppg)
- [[71](https://github.com/braedon/prometheus-es-exporter/pull/71)] Add metric for index aliases

Fixes:
- Fix issue where main config file is required even when queries are disabled

Enhancements:
- [[68](https://github.com/braedon/prometheus-es-exporter/issues/68)] Add a metric for the count of mapped fields by index and field type

Enhancements:
- [[63](https://github.com/braedon/prometheus-es-exporter/issues/63)] Enable loading all configuration options from a file (thanks RobertKirk)

Enhancements:
- [[60](https://github.com/braedon/prometheus-es-exporter/issues/60)] Add new metrics for the three cluster/index/shard status colours

Breaking Changes:
- [[38](https://github.com/braedon/prometheus-es-exporter/issues/38)] Drop old metrics on query failure (behaviour configurable via [`QueryOnError` config](exporter.cfg#L17))
- [[57](https://github.com/braedon/prometheus-es-exporter/pull/57)] Drop old metrics that are missing from subsequent query runs (behaviour configurable via [`QueryOnMissing` config](exporter.cfg#L23))

Enhancements:
- Add support for python 3.7
- Optimise Dockerfile