mbed new should fetch the latest stable release, by default

[AlessandroA]

At the moment if I run

$ mbed new myapp

the tool fetches the HEAD of ARMmbed/mbed-os master, which is prone to bugs and instabilities.

A sensible default would be to fetch the latest stable release/tag.

See for example: ARMmbed/uvisor#312.

@screamerbg @geky @sg-

[sg-]

tag "latest" created. mbed CLI can now pull against this.

[screamerbg]

Thanks. Will be introduced in the next mbed CLI release

[screamerbg]

This is now supported in the latest version of mbed CLI (0.9.5)