Create module to automate shell installation

[twolfson]

I would like to make shell completion automatically installed via a postinstall script. The goal is to have something that is expected to work nicely with commander-completion. It should have ample tests for bash and zsh.

A working version of the tab completion logic can be found in bin/foundry/completion but it doesn't place nice with zsh in Vagrant =(

[twolfson]

Since we are moving to local modules in #18. This would be more fitting in the foundry-cli module.