Increment inspector port for spawned subcommands

[shadowspawn]

Look for node inspector arguments used for configuring debugging and increment the port number. This avoids launch errors when debugging git-style subcommands from top-level command.

A call like this which implicitly uses default port of 9229

node --inspect pm.js sub

will spawn a sub-command with port number incremented:

node --inspect=127.0.0.1:9230 pm-sub.js

See earlier pull request with research and comments about approach of incrementing ports: #874

Related issues: #533 #838

Code can be reviewed by interested readers.
I have not minimised number of regex calls, could preflight with a simple search?
Opening as draft pull request because no changes yet to README or tests.

[shadowspawn]

I have added tests, and check for --inspect before trying regex.

(I will update README after decide #990 as I moved --harmony in that PR.)

[shadowspawn]

Added section to README. All done. Ready for review.

[abetomo]

Thank you.

[shadowspawn]

Available now as a prerelease. See #1001

[shadowspawn]

Shipped in v3: https://github.com/tj/commander.js/releases/tag/v3.0.0