get return code of shell command

[le91688]

Currently, adb.shell("my command) returns the result string.

getting the command return code can be achieved by doing something like

adb.shell(my_command; echo exit_code: $?) and parsing the return code out, but would be preferable to have a better mechanism for getting the return code ... possibly like

adb.shell(my_command, ret) where ret is assigned the return code