Add -o option to open session files

Open/Create option:
Discussion should arise here on wether one option should only edit or
both edit/create session files. one could also have a -n(new) option.

Note on example files: having an examples folder available would greatly
help when creating new sessions, -n option could receive an
example_session_name which would copy said template in order to rapidly
get new projects up and running.
examples could be under $TMUXSTART_DIR/examples as stated in https://github.com/treyhunner/tmuxstart/pull/12

This commit also: 
- Moves code from inside the main loop to local functions to keep things
more organized.
- Renames usage function to _usage to be consistent with other local
funcitons and updates it's output