Add amixer --view, to filter playback, capture or all

[GitHub issues - opened <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-utils issue #67 was opened from jidanni:

alsamixer has
```
       -V, --view <mode>
              Select the starting view mode, either playback, capture or all.
```

Perhaps amixer could also have something similar,'
e.g., so that
```
$ amixer --view capture
```
would only dump the capture related settings.
Currently one could say that
```
$ amixer --view all
```
is the default.

Issue URL     : https://github.com/alsa-project/alsa-utils/issues/67
Repository URL: https://github.com/alsa-project/alsa-utils