Add option for setting the current platform

[atanasovg]

Currently, most of the commands require the platform-specific parameter, e.g.

tns prepare android
tns build android

What I suggest is to enable some way of setting the current platform and then to call all the commands without this additional parameter. Something like:

tns platform setCurrent android
tns prepare //(uses Android)
tns build //(uses Android)

[ligaz]

What about just adding the capability to execute the commands without platform and it will execute the command for all added platforms.

$ tns platform add android
$ tns run

will run (prepare, build, deploy) for Android.

$ tns platform add android
$ tns platform add ios
$ tns run

will run for both platforms.

[atanasovg]

@ligaz Sounds good

[teobugslayer]

Related to #475

[ligaz]

Superseded by #475