Scripts

These series of patches
1) refactors out script processing
2) supports login and interactive scripts (~/.plush/profile and ~/.plush/env)
3) auto creates those scripts if they don't exist
4) implements more of the shell startup sequence (processing login, scripts, etc...)
5) handles PS1 in terminal mode

Warning: After this set of changes, plush will try to read /etc/profile if it is a login shell - your system probably has one, and it probably makes use of features that Plush can handle yet. This is okay, but error messages will display when you launch.