Add support to clear custom dimensions

[markkuhn]

Issue #, if available: #132

Related:

• Add support to clear custom dimensions and configure flush behavior on dimensions aws-embedded-metrics-java#105
• Support clear custom dimensions and related enhancements aws-embedded-metrics-python#89
• Add support to clear custom dimensions aws-embedded-metrics-dotnet#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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