Add tab-completion of positional and named values

Followup of #281 in support of #175.

For example assume we have:

``` dart
@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:

``` dart
@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
```