allow number parameter for --head and --tail

Hey! I love aura and hope this is a welcome contribution!

This allows one to specify a specific number of items to include in the `--head` and `--tail` options to the search command. This change is backwards-compatible in that the number parameter is optional. If the flag is called in the same way as it was before this change, the number is still assumed to be 10.

In other words, the following continues to work, showing the first 10 results:

```
$ aura -As --head needle
```

The following now works as well, showing the first 3 results:

```
$ aura -As --head=3 needle
```

The reason I made this was because the default value of 10 always scrolled the result I was interested in past my terminal height, since usually what I'm looking for is one of the first if not the first result.

It's working nicely :)