Add helper to check for git

[coolaj86]

We should have some helper functions that can check for system-level dependencies and be called in the __init_xxxx of any installer, and print out a helpful message if that tool isn't there.

We've partially done with this git, but it should be more universal - in almost every vim-* installer, for example.

_webi/template.sh (? or maybe bootstrap? I don't recall)

function webi_check_git() {
    # do the check / print the message
}

vim-prettier

function __init_vim_prettier() {
    webi_check_git
    # do stuff...
}