Add showcompact method for vectors of Number's
Print the [] for a vector of length 0

```jl
julia> showcompact(rand(5))
[0.335052,0.678365,0.187975,0.345677,0.981752]
julia> showcompact(Float64[])
[]
```
Merge pull request #4690 from JuliaLang/db/showcompactvector

Add showcompact method for vectors of Number's