Add tab-completion of positional and named values

[seaneagan]

Followup of #281 in support of #175.

For example assume we have:

@Task()
bump(
    @Positional(allowed: const ['major', 'minor', 'patch', 'build', 'release'])
    String releaseType) {
  new PubApp.global('den').run(['bump', releaseType]);
}

Then the user should get the following completions:

grind bump:[TAB]
major
minor
patch
build
release

And similarly for options:

@Task()
bump(
    {@Option(allowed: const ['major', 'minor', 'patch', 'build', 'release'])
    String releaseType: 'patch') {
  new PubApp.global('den').run(['bump', releaseType]);
}

grind:bump --release-type [TAB]
major
minor
patch
build
release