Add optional JSON output to the search binary

[neongreen]

It'd be rather useful if zoekt accepted a -json switch that would instruct it to output search results in a machine-readable format looking roughly like this:

[
    {
	"FileName": "repo/file.txt",
         "Repository": "repo",
         "LineMatches": [...], // array of objects
         ...
         ]
    },
    ...
]

Would you accept a PR to this effect?

[hanwen]

sure. Look at CONTRIBUTING for instructions on sending code.