adding a shell modus to the cli

this allows to reuse the running client
but also required extra io handling beyond the stdin/stdout

all in all a bit of a side-track to the ongoing work for brianhealey#16

but lays down the ground for adding the remaining actions

example.ampsh

@@ -0,0 +1,4 @@
+#! /usr/bin/env -S pyamplipi sh
+
+grp ls
+src ls