CloudWatch Metrics Input Plugin to Support Percentile Based Metrics

## Feature Request

Opening a feature request kicks off a discussion.

### Proposal:
The CloudWatch [GetMetricData](https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/API_GetMetricData.html) API call has the capability to use an [Expression ](https://docs.aws.amazon.com/sdk-for-go/api/service/cloudwatch/) to allow for gathering pXX.XX (IE p95) statistics metrics.

### Current behavior:

Based on [this](https://github.com/influxdata/telegraf/blob/master/plugins/inputs/cloudwatch/cloudwatch.go#L412) function, only 'minimum, maximum, sum, samplecount, average' are supported metric values. There is no extendedmetrics available which would be used by extended metrics.

### Desired behavior:
Allow for pXX.XX to be valid metrics to be gathered by the CloudWatch input plugin.

### Use case:
For statistical analysis, it is often better to throw out outliers of a dataset to keep from distracting from underlying trends. Using p95 allows for a higher confidence interval in what real "max" usage is, p5 similarly for "minimum" usage, and p50 for "average" usage. This allows for a statistics_input object to contain arbitrary pXX.XX values and return them from the GetMetricData API. This is suggested as an additional option, rather than complete replacement, to allow for users of this plugin to choose which statistic set they prefer.

Start with the CloudWatch input plugin in cloudwatch.go around line 412 and review the AWS GetMetricData API and Expression documentation linked in the issue. Determine how percentile statistics such as p95 can be accepted alongside the existing metrics, then verify that the plugin returns those values without removing current statistic options.

Written by the indexing model from the issue text.