Norm along side axis

It would be nice if the `LinearAlgebra.norm` function have the `dims` keyword, like others linear algebra functions and like numpy norm axis parameter (https://docs.scipy.org/doc/numpy/reference/generated/numpy.linalg.norm.html)

Just for example:
```
function norm(itr, p::Real=2; dims)
sum(itr.^p; dims=dims).^(1/p)
end
```