Add JSON log parser

[yngwiewang]

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

I've made various changes:

Bump the Go version to 1.13 in Dockerfile
Add a new flag and config-file argument "-parser"
Abstract a generic Parser interface
Provide two implementations of Parser interface: plain text parser TestParser and JSON parser JsonParser

Very welcome to propose improvements to the code.

[martin-helmich]

Thanks for the PR! As already said, 👍 on the general design. Will review in detail ASAP.

[martin-helmich]

Sorry for taking so long. A big 👍 in general on this change; I've just added a few notes and thoughts -- let me know what you think. ;)

[yngwiewang]

I'm not a GitHub expert. I should also modify textparser_test.go and loader_flags.go. Should I make all the modifications above in my local repo and commit once and push again, or some other operations? Please give me some advice.

[martin-helmich]

Should I make all the modifications above in my local repo and commit once and push again, or some other operations? Please give me some advice.

Just commit as it is most convenient for you 😄 -- I'll probably squash the entire branch into a single commit when merging, anyway, so you don't need to do any Git history rewrites or any such things before pushing.

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