Allow empty (but labelled) annotations
In combination with #51, the following will be possible:

```
task 42 mod +\#52
task 42 annotate github:
```

If the part after the label is empty, taskopen should add the task's ID instead as an argument to the executed command, so that:

```
taskopen -x github.sh 42 \\github
```

executes `github.sh 42`, which then queries taskwarrior in order to get the project name, ticket number and assemble the corresponding URI.
