stdm with dims arg is missing

[denizyuret]

https://github.com/JuliaLang/julia/blob/2f426e45c24e0d94c11f503aa696a902f1bb6aed/stdlib/Statistics/src/Statistics.jl#L381 should also have a dims keyword argument.

[nalimilan]

You can use the mean argument to std. stdm is basically deprecated, except that we can't do that before 2.0. The docstring should probably point to std.

[denizyuret]

Currently the mean argument has a problem: https://github.com/JuliaLang/julia/blob/2f426e45c24e0d94c11f503aa696a902f1bb6aed/stdlib/Statistics/src/Statistics.jl#L338 uses something(mean, Statistics.mean(A, dims=dims)) which ends up calculating the mean whether or not the user provides one (both args are evaluated afaik). Typically people would use the mean option not to calculate the mean twice.

[nalimilan]

Indeed, good catch. Fortunately that's easy to fix, just replace this with an if mean === nothing block. Would you make a pull request?

[nalimilan]

Fixed by #68.