Allow customizing the default branch of the generated git repository

[mtfishman]

It looks like PkgSkeleton.generate is hard-coded to generate git repositories with master as the default branch. It would be nice to be able to customize that, and additionally use git config --get init.defaultBranch as a default value.

[mtfishman]

This may require using Git.jl, as discussed here: https://discourse.julialang.org/t/how-to-change-branch-name-using-libgit2/84353