Add support for := separator (#68)

[eliangcs]

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[eliangcs]

@fogine would you like to review this PR? Thanks!

[fogine]

@eliangcs , overall this feature seems to be well implemented. I've stumbled upon small issue with exporting json string values (fix provided in PR). Everything else works as expected.

Before this gets merged, I'd like to propose a change for used json parser though.
Please, take a look at json5 extension - respectively python-json5. It'd make work with json inside http-prompt much more enjoyable as it eases some strict rules.

Thanks!

[eliangcs]

@fogine json5 support is nice, but let's do it in another PR. You're welcome to create a PR if you have time to do it :)

If you were to implement json5 support, instead of using pyjson5 as a hard requirement, make sure it uses pyjson5 as a fallback of Python's builtin json, i.e., use pyjson5 only when Python's builtin json fails. pyjson5 does not seem to well maintained (last release was in 2015), so I don't want to rely on it too much.