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.
setup `golint` to run on your code. 

> this comment should start: PathRoot ...

lots of the code isn't linted properly, but trying to make things better :smile: