Use an interactive shell for running custom commands

Also, use the user's shell (from the SHELL env variable) instead of bash. Both
of these together allow users to use their shell aliases or shell functions in
the interactive command prompt.

pkg/commands/oscommands/os.go

@@ -35,11 +35,13 @@ type OSCommand struct {
 
 // Platform stores the os state
 type Platform struct {
-	OS              string
-	Shell           string
-	ShellArg        string
-	OpenCommand     string
-	OpenLinkCommand string
+	OS                  string
+	Shell               string
+	InteractiveShell    string
+	ShellArg            string
+	InteractiveShellArg string
+	OpenCommand         string
+	OpenLinkCommand     string
 }
 
 // NewOSCommand os command runner