Machine-parseable error reporter

[zkat]

Might be nice to have a reporter that's able to output errors as JSON.

Ref: #28 (comment).

[dvermd]

Hi, I've started working on a JSONReporter.

Were you thinking of pulling serde_json dependency or going to a hand crafted serialisation ?

[zkat]

I think doing hand-serisalization is sufficient in this case, and it avoids an extra dependency. What do you think?

[dvermd]

I think hand-written serialization will be good enough

[dvermd]

I've got most of the serialization done. Only SourceCode is left.
Do you think SourceCode should made it to a machine-writable format ?

In case, yes:
Both NarratableReportHandler and GraphicalReportHandler use the same render_snippets function that calls render_context function which each reporter has its own.

A utility function can be extracted from render_snippets and be used in the 3 Reporters but I wonder where would be the best place to put this function.

I don't think the creation of the JSONReporter has its place in the MietteHandlerOpts builder. It can be instantiated directly or with its own builder to provide context_lines. What's your view on that ?