Add bash history saving and restoring (first version).

This does not yet have flag to turn the feature off.
Extract `save_shell_history` to a separate function
Update changelog
Flag gate the bash history restore feature
Document bash restore history feature
Update the readme
Merge pull request #49 from tmux-plugins/bash_history

Bash save and restore history feature
Merge remote-tracking branch 'upstream/master'

Conflicts:
	scripts/restore.sh