use unix tools if available in windows

[1000i100]

I use git-bash (like probably nearly all windows npm user).
if i run mkdir -p plop it create me plop folder if not existant and do nothing if it exist.
if i do the same as npm task or with npm-run-all it create me -p folder and plop if they don't exist and fail if there already there.

is'nt possible to use git-bash command in priority when available ?

(copy from mysticatea/npm-run-all#145 )