[Feature Request] Implement `LogScalable` for `usize` and `isize`

[saona-raimundo]

What is the feature ?

The LogScalable trait describes types that can be represented in logarithmic scale.
It is currently implemented for uN, iN and fN types, but not for usize and isize.

https://docs.rs/plotters/0.3.5/plotters/coord/combinators/trait.LogScalable.html

Why this feature is useful and how people would use the feature ?

I was plotting some data that was usize and thought that it could be added. It is simply to have less compile errors when using the crate.

Additional Information

The PR would be two lines of code in
https://docs.rs/plotters/0.3.5/src/plotters/coord/ranged1d/combinators/logarithmic.rs.html#44
I am happy to do it.

[AaronErhardt]

Thanks for the report. I think this would indeed be a useful addition.

I am happy to do it.

If you could submit a PR, that'd be great :)