Discover and inject task parameters

Followup of #281 in support of #175.

`@Task` annotated methods will be able to use the same exact API as [unscripted](https://github.com/seaneagan/unscripted) to define method parameters that represent positionals, options, and flags, and have those discovered and injected with command-line arguments.

For example:

``` dart
@Task()
bump(
@Positional(help: 'The type of release being performed.', allowed: const ['major', 'minor', 'patch', 'build', 'release'])
String releaseType),
{@Option(help: 'Do a pre-release with this prefix e.g. "dev"')
String preId,
@Flag(help: 'Whether to do a pre-release')
bool pre} {
var args = ['bump', releaseType];
if (pre) args.add('--pre');
if (preId != null) args.addAll(['--pre-id', preId]);

new PubApp.global('den').run(args);
}
```

As in unscripted, all metadata (`Positional`, `Option`, `Flag` and their parameters) is optional.

This will require a lot of the same code existing in unscripted, so I may need to factor that out into a shared library or package along with some of the code that went in in #281.