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.
let's make this just `IPFS_DIR`, and a proper constant:

```Go
const EnvDir = "IPFS_DIR" 
```