Add option to disable fetching on branch creation

[TamaMcGlinn]

I had this code commented out for the past half year; time to formalise the option so others can use it.

For me, an additional fetch is really annoying because it adds more than 10 seconds to this workflow which I use multiple times per day, and I already have a different binding to fetch explicitly.

Default behaviour unchanged; add this to your config to skip fetch (also updated the docs):

require("git-worktree").setup({
    fetch_on_create = false
})

[TamaMcGlinn]

This fixes #85

[HoffsMH]

would love to see this, for those of us who have our ssh credentials on something like a yubikey that requires a touch to activate this sort of thing can get annoying

[TamaMcGlinn]

@ThePrimeagen could you please merge this?