[TW-1727] Overzealous alias expansion

_Daniel Shahaf on 2015-11-18T21:24:42Z says:_

```
% task add find a shell at the beach
Created task 1.
% task _get 1.description
find a exec tasksh at the beach
```

The word `shell` should not have been subjected to alias expansion, since aliases are for commands only (according to `taskrc(5)`) and the command is `add`.

Yes, using a `--` guard inhibits alias expansion, but that is not a solution because a user might call `task add foo bar` without realizing that one of the words in his command line is a configured alias: from that user's perspective, `task` will have silently corrupted his input. (That's what happened to me, and only thanks to keeping ~/.task in git could I restore the original task description I had added.)