Refactor grinderArgs()

[seaneagan]

The purpose of grinderArgs() it is to be able to access custom command-line arguments from tasks. However, package:args currently doesn't allow unknown options to be passed anyway (arg parsing will fail). Ideally a task could declare its own custom command-line options and flags. That would allow:

• including help text for them via grind -h
• validating custom command-line arguments with task arguments.

It would be really cool to reuse unscripted's Flag and Option annotations:

@Task('Deploy the project.')
deploy(
  @Option(allowed: const ['local', 'ci', 'test', 'prod'], help: 'Which environment to deploy to.')
  String env) {
  // ...
}

Then grind -h would include something like:

[deploy] Deploy the project.
  --env    Which environment to deploy to.

If we do keep grinderArgs(), I think it should be a getter, and probably move it to GrinderContext, or integrate with whatever solution we do for dart-lang/core#31.

[devoncarew]

grinderArgs is mostly used to be able to locate the dart sdk from tasks. If we're not able to discover it via the current path (or which), then we fall back on looking for a --dart-sdk cli option.

Refactoring it's meaning sounds good. We may want to hang the cli args on an instance of a Grinder object though, instead of using a global var. That was an expedient solution -

[seaneagan]

Also, I think it'd be great to support positional arguments like this:

@Task('Deploy the project.')
deploy(
  [@Positional(allowed: const ['local', 'ci', 'test', 'prod'], help: 'Which environment to deploy to.')
  String env = 'local']) {
  // ...
}

Which can be used in the CLI as:

grind deploy (defaults to 'local')
grind deploy:prod

[seaneagan]

I'll take this. I'm going to add support for positional args first e.g. grind bump:patch. Those are much more straight-forward since they are syntactically attached to individual tasks. For named args we'll need to decide whether to support something similar e.g. grind bump:type=patch:pre=true or traditional named args e.g. grind bump --type=patch --pre