Rework config file location

Implement IPFS_CONFIG_DIR and refactor code to make it simpler. Fix bug in `-c` option handling in `init` subcommand, and add `-c` support to `config` subcommand.

Also add `ipfs init` `-d` option to change default datastore location when initializing ipfs.
Keep the name `Filename`. this is idiomatic go. within the package, it makes sense. outside the pacakge, it's used as `config.Filename()`