feat: add --build-process-tree feature

[isaacs]

This allows a user to build the processinfo temp files, without actually
dumping the process tree to stdout.

The goal is to use this in node-tap to see which processes covered which
files, so we know which tests to re-run on changes to the source files.

[coreyfarrell]

@isaacs this produces a parse error in test/nyc-integration.js. Would you mind moving the describe('--build-process-tree', function () { out of the describe('--show-process-tree', function () {, then add the closing '}) after your new test (currently you're not closing the added describe function).

[isaacs]

@coreyfarrell Oh, oops, some rebase scar tissue got in there, how embarrassing. Fixed and force-pushed.

[coveralls]

Coverage increased (+0.007%) to 96.304% when pulling efcd6cd on isaacs:build-process-tree into 364eb33 on istanbuljs:master.

[coveralls]

Coverage increased (+0.007%) to 96.304% when pulling efcd6cd on isaacs:build-process-tree into 364eb33 on istanbuljs:master.

[coveralls]

Coverage increased (+0.007%) to 96.304% when pulling efcd6cd on isaacs:build-process-tree into 364eb33 on istanbuljs:master.

[coveralls]

Coverage increased (+0.007%) to 96.304% when pulling 2edc77b on isaacs:build-process-tree into 364eb33 on istanbuljs:master.

[JaKXz]

@coreyfarrell

Thanks @isaacs I was just curious!