A new CLI command to check system configuration

[teobugslayer]

It is beneficial to have a dedicated command in the CLI which checks current system configuration of user's machine and prints warnings about missing or mis-configured dependencies.

1. We are already doing this during installation but configuration can change
2. This should be implemented for both CLIs
3. Add additional checks: ANDROID_HOME, JAVA_HOME, ANT_HOME. See here for inspiration.

$ tns doctor

as possible command name?