Add package manager commands

When you have a package manager installed, it may be a different version than rundev manages for you. So a `rundev pm [args]` command would be helpful to use the same package manager.

It can also detect package installing commands so the package manager doesn't have to run twice, one for install and one for dev server restart, the second one can be omitted.

A similar `rundev node` command could be also provided, for consistency.