Suggestions re ipfs repos

- I assumed that `iim` just installed the binaries, and used the global defaults re repo options. Not so-- `iim` uses its own repos
- i guess this is because there are incompatibilities across ipfs versions and ipfs repo versions that we want to side-step w/ use cases that require switching between ipfs versions often (makes sense)
- [x] Maybe warn the user that the repo used will not be the default but instead be `iim`'s own version -- maybe during install?
- [x] Maybe add a line to https://github.com/alanshaw/iim#how-does-it-work saying how repos work?
- [ ] Maybe provide an `iim use` option/flag that configures the `ipfs` wrapper to use the defaults (global location, respect `$IPFS_PATH` in env)? perhaps `iim use go --default-repo` or `iim use go -c $IPFS_PATH` ?

See also: #5