[2.1] Add headers support

[rez1dent3]

Development of header transfer functionality:
#41
tokopedia/gripmock#144

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Before release:

• Human-readable input/headers;
• Update readme.md;
• Update documentation, add headers;
• Test the ranking of stubs;

[rez1dent3]

I thought about it and I don’t really like the approach of moving headers to a separate input key. I'm thinking about other options.

There is no better idea, I'll leave it as it is.

[rez1dent3]

tokopedia/gripmock#140