Add option to prefer outermost project

For some configurations it makes more sense to relate opened files to the topmost project type.

In my case I rarely (if ever) change anything in a subproject without considering it to be part of its superproject and then it can become cumbersome to have all files in my subprojects (generally git submodules or Erlang rebar dependencies) as parts of separate projects.

This is also a better way to solve the issues I tried to solve with https://github.com/jrockway/eproject/pull/43 so I'll close that.