tsc CLI: allow boolean options to have explicit false value

[alexeagle]

I have some .d.ts files referenced in the project (from Angular 2) which are not compatible with --strictNullChecks.
I want to enforce strict nulls within my own project though. So I have tsconfig.json with 'strictNullChecks': true specifically to direct my editor to underline these.
In our google-internal build system, we have our own tsc wrapper that does specific type-checks (and uses a different generated tsconfig.json so this all works fine in the build.
But, users benefit from being able to just run a vanilla tsc command on the command line to diagnose setup issues.

So, I'd like to be able to override any setting in the tsconfig.json with a command-line option. In some cases I can do so (eg --target=es5 overrides 'target': 'es6') but with boolean options only the true value can be passed, eg --strictNullChecks

I propose --strictNullChecks=false be accepted on CLI

[evmar]

(In case anyone else is wondering, I checked the tsc source and it appears that there's no way of passing in false, such as --noStrictNullChecks.)

[mhegazy]

we can add a look-ahead for boolean-based flags, and consume the result iff it is true|false.

[RyanCavanaugh]

Accepting PRs for this

[mhegazy]

thanks @HerringtonDarkholme!