Introduce settings file

[mattfarina]

Two things are prompting this:

• We introduced an aggressive caching strategy and almost immediately got an issue on it (Glide gets dependencies in vendor and in $GOPATH/src at the same time #127). Should it put a copy in the GOPATH, a separate cache (important for forks), or not at all? And, what should the default be? I'm not sure there will be a consistent answer. Am adding flags to control this behavior. Would be useful to have a config file (like git) for users to control this.
• If we already have a $HOME/.glide folder (needed for the cache) than this become easier.

Considering this for the 0.8.0 release. Feedback welcome.