[Feature Request] Add a convenience macro for making Polynomials

It would be nice if there where a convenient way to make a Polynomial without having to write out every time "Polynomial"

For instance, including something like:
```
macro P_str(s)
Polynomial(eval(Meta.parse("["*s*"]")))
end
```
Makes it easy to make a Polynomial like so:
```
julia> P"1,2,4,5,6"
Polynomial(1 + 2*x + 4*x^2 + 5*x^3 + 6*x^4)
```