move config package default values to new config/default package

go-ipfs version: 0.4.14-rc1-5622cc5bb
Repo version: 6
System version: amd64/darwin
Golang version: go1.10

Type: Code Refactoring

Description:
1) repo/config/bootstrap_peers.go
2) repo/config/init.go
3) repo/config/version.go

These files under package `config` use a lot of default values which users might want to customize for themselves.

I think a better design would be to move these default values to a new package `config/default`. So that user has one place to go if they want to tweak something.

I know this is a minor thing but might be useful for users and developers for future maintenance. If you want I can work on this and make a PR.