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/dummies.go

@@ -51,11 +51,13 @@ func NewDummyCmdObjBuilder(runner ICmdObjRunner) *CmdObjBuilder {
 }
 
 var dummyPlatform = &Platform{
-	OS:              "darwin",
-	Shell:           "bash",
-	ShellArg:        "-c",
-	OpenCommand:     "open {{filename}}",
-	OpenLinkCommand: "open {{link}}",
+	OS:                  "darwin",
+	Shell:               "bash",
+	InteractiveShell:    "bash",
+	ShellArg:            "-c",
+	InteractiveShellArg: "-i",
+	OpenCommand:         "open {{filename}}",
+	OpenLinkCommand:     "open {{link}}",
 }
 
 func NewDummyOSCommandWithRunner(runner *FakeCmdObjRunner) *OSCommand {