Add support to clear custom dimensions

[markkuhn]

Issue #, if available: #132

Related:

• https://github.com/awslabs/aws-embedded-metrics-java/pull/105
• https://github.com/awslabs/aws-embedded-metrics-python/pull/89
• https://github.com/awslabs/aws-embedded-metrics-dotnet/pull/38

Description of changes:

• Added a resetDimensions() method for clearing custom dimensions
• Added a field to MetricsLogger to configure flush() to preserve/not preserve custom dimensions
• adjusted README to reflect the correct behaviour of flush(), resetDimensions and setDimensions()

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