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And refactored a few names/comments

- safer parsing
- multiple inputs
- add: dup detection
- common cfg writing

cc @mildred

Removed config.Filename and replace it with config.GetConfigFilePath that
takes a configuration directory as argument instead. Makes code simpler.
ipfs.getConfigDir now also return the default configuration dir instead of
an empty string in some cases.

WARNING: change breaks old configs.
@whyrusleeping @perfmode

This commit changes the way addresses are stored in config files.
It lumps Identity.Address and RPCAddress into Addresses. This
commit also fixes several golint issues.

There are (so far) two sorts of addresses that peers care about:

- peer network addresses, local to listen, saved to bootstrap.
  `config.Identity.Address`

- peer RPC network address, local RPC tcp endpoint
  `config.RPCAddress`

@whyrusleeping @perfmode

also replace *Datastore with Datastore

Adds "ipfs config" command designed in git style.
See specific config's values with:

        $ipfs config datastore.path
        ~/.go-ipfs/datastore

Assign a new value with:

        $ipfs config datastore.path ~/.go-ipfs/datastore

Open the config file in your default $EDITOR:

        ipfs config edit