Implement XDG base directory specification (issue #120)

[mildred]

This is how it has been implemented:

• Unless specified using the -c command line option, the configuration is generally found at ~/.config/go-ipfs/config
• The -c command line option now specifies the configuration file itself (and not its directory)
• During ipfs init it is possible to specify the database location using the -d option.
• The database location generally defaults to ~/.local/share/go-ipfs/datastore
• If a relative path is found in the configuration file for the database location, this file is relative to the XDG Data directories.

What might be missing is transition code to move/read files already in ~/.go-ipfs

[jbenet]

@mildred thanks but i do not want the XDG format. I looked in depth at it, and disagree with it. Sorry, you'll be able to do exactly this by setting an env variable (or passing in -c) and modifying the config file.

[jbenet]

Spelling out how:

1. use ipfs -c ~/.config/go-ipfs
2. IPFS_CONFIG=$HOME/.config/go-ipfs (this needs implementing)
3. change the Datastore location in the config file to point to ~/.local/share/go-ipfs/datastore