handle multi process output comparison cleanly

Nobody has claimed this yet.

lately I've been having to write code like this:

``` js
function compareStatuses (t, dat1, dat2) {
// get dat1 'dat status' json
var st1 = spawn(t, dat + ' status --json', {cwd: dat1, end: false})
var status1

// get dat1 'dat status' json
st1.stdout.match(function (output) {
try {
JSON.parse(output)
status1 = output
return true
} catch (e) {}
}, 'status 1 is json')

// get dat2 'dat status' json, should be equal to dat1
st1.end(function () {
var st2 = spawn(t, dat + ' status --json', {cwd: dat2})
st2.stdout.match(function (output) {
try {
JSON.parse(output)
t.equal(status1, output, dat2 + ' status should match ' + dat1)
return status1 === output
} catch (e) {}
}, 'status1 matches status2')
st2.end()
})
}
```

it would be nice if:
- instead of putting the JSON.parse in a try/catch there was a builtin json specific thing that was a one-liner
- instead of having to do `end: false` on the first one, and put the 2nd one in the `.end` of the first, if the flow control provided by tape-spawn was more elegant
- when the test fails it doesn't print both sides output, because the 2nd assertion is not aware it is doing a comparison. I could put it in the `t.equal` I added in to the 2nd one, but that makes the output noisy when it passes

API suggestions welcome

Start by locating the tape-spawn implementation and tests for stdout matching, process completion, and assertion reporting. Compare those entry points with the three requested behaviors: JSON-aware matching, cleaner sequencing of spawned processes, and comparison-aware failure output. Done means an agreed API with coverage for each behavior.

Written by the indexing model from the issue text.