Split concepts of "env" and "rc" into separate files

Opinions differ, but [many agree](http://superuser.com/a/183980/152088) that environment variables and aliases/functions/prompt settings belong in different files (`.bash_profile` and `.bashrc`, respectively, for `bash`).

This pull request introduces this concept into the config, distinguishing between `rc` ("source this file") and `env` ("export this variable"). It also adds a utility function, `sprinter_prepend_path`, sourced at the top of the `env` section, which enables the various `PATH` exports to avoid duplicating entries.

**TODO**
- [ ] Improve tests related to functional split
- [x] A bit more local verification