Add CLI switch to flores init to allow for existing base dir

[kokkonisd]

Currently, the init subcommand has a sort of "safety feature" where it will decline to initialize a default site given a non-empty directory. I think that's still useful, because it prevents the user from accidentally destroying a directory. However, there are cases where you might legitimately want to do that; init should have some sort of switch (maybe -f/--force?) to initialize a site even if the directory exists, without destroying the current content of the directory (only adding what's necessary for the site to function).