Enable user-defined actions

Also fix pclose() handling within Info plugins,
and prevent them from screwing up the terminal with error output
on initialization.

This is still rather crude, but at least it's possible.

nncmpp.adoc

@@ -85,6 +85,18 @@ To adjust key bindings, put them within a *normal* or *editor* object.
 Run *nncmpp* with the *--debug* option to find out key combinations names.
 Press *?* in the help tab to learn the action identifiers to use.
 
+You may also define and bind your own actions, launching arbitrary
+shell commands.  Note that you cannot override internal actions in this manner.
+
+....
+actions = {
+  "pioneer-on-off" = {
+    description = "Pioneer amplifier: turn on/off"
+    command = "elksmart-comm --nec A538"
+  }
+}
+....
+
 Spectrum visualiser
 -------------------
 When built against the FFTW library, *nncmpp* can make use of MPD's "fifo"