Adds `$FORGIT_SWITCH_AUTO_CREATE_BRANCH` and `$FORGIT_CHECKOUT_BRANCH_AUTO_CREATE_BRANCH`
environment variables that can be set to `false` to prevent `checkout_branch <BRANCH_NAME>` and
`switch_brach <BRANCH_NAME>` from creating new branches when they do not exist. With the environment
variables set to `false`, new branches can still be created explicitly using
`switch_branch -c <BRANCH_NAME>` and `<checkout_branch> -c <BRANCH_NAME>` respectively.
