Support multiple shells

Related to #8 issue
switch to last buffer not first index 1 buffer

Suggestion by kyagi
Merge pull request #12 from kyagi/multi-shell

Support multiple shells
Update README.md
Reduce setq expression
Separate functions for readability
Use 'when' instead of 'if' if there is no else clause
Merge pull request #13 from kyagi/refactoring

Refactoring
Bump up version