Added special ALL and MISSING values for ``asv run``

@mdboom - just an idea, so feel free to close without merging. This makes it easy to do:

```
asv run ALL --steps=20
```

Interestingly, this opens up the option to do:

```
asv run MISSING --steps=20
```

as part of the nightly cron job, which will slowly fill in all the missing commits for a project (can be done in addition to ``asv run NEW``)