Marshal method for HCL Parser

[amarlearning]

Changes:

1. Implemented Marshal method for HCL Parser.
2. Updated the unit test for the same.

[amarlearning]

@knadh ready for review :)

[knadh]

Thanks for the PR @amarlearning. Please see:

• fix(hcl): Flush bufio buffer to backing buf #135
• HCL2 <-> JSON Transformer? hashicorp/hcl#294

HCL:JSON (and other schema) do not map 1:1 due to differences in semantics, which means if you read from a JSON config and marshal to HCL, what comes out may be different, which can be dangerous. That's why HCL marshalling has been left unimplemented.

[amarlearning]

@knadh have a look at this viper implementation, they also did it but in a slightly different way

[knadh]

I don't think that addresses the incompatibility between JSON and HCL specs.

Came across this: spf13/viper#967