Add JSON parser to support JSON log files

[yngwiewang]

[FEATURE] Add JSON parser to support JSON log files
Fixes: #793

I've made various changes:

1. Bump the Go version to 1.15
2. Add a new flag argument "-parser"
3. Abstract a generic Parser interface
4. Provide two implementations of Parser interface: plain text parser TestParser and JSON parser JsonParser

Very welcome to propose improvements to the code.

[qlty-cloud-legacy]

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