Add a new public method runCommand(), more generic than runInstall(), that uses an hardcoded 'install' in the command to launch

Hello,

Now, until 3 days, as the `runInstall()` is public (nice to be able to run it), why not add also a more generic public method that can run any command ?

The fact is, with `runInstall()`, it seems to use an hardcoded word which is `install` as first param for the package manager.
It would be nice to have an handy function in yeoman-generator to run any command (not only a package manager as` npm` or `bower`), with any params.

This PR shows the spirit of the modification it could be

Thanks for your feedback