Sometimes angle units are dimensionless - new system, more general equivalency?

Angle units are formally dimensionless (or more precisely, in SI at least radians are a derived unit), and while for clarity it is definitely good not to always treat them as such, sometimes it would be handy.  E.g., suppose I have defined `Omega = 2.*np.pi*u.rad/P`, then I'm all set up to do `phase=Omega*t`, but I get a unit exception if I try
```
I = 1.4 * u.M_sun * (10*u.km)**2
Erot = 0.5*I*Omega**2
Erot.to(u.erg)
```
Would it be possible to, sometimes, tell that one just wants to use `rad=1`? It seems equivalencies are rather cumbursome for this, but are there other solutions? Perhaps an `si_rad_is_dimensionless` system? Or a `to_system(rad=1)`?

(Note, there is some relation to my thinking on having a `u.pi` -- #1160 -- which is useful when one is thinking of orbital phases.)