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

[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.