Add before/after hooks

[DannyBen]

cc #359, #116

• Remove unused --help related code from the generated run() function
• If src/before.sh exists, it will be put into a before_hook() function and called in run() after processing the args, but before running any of the commands.
• If src/after.sh exists, it will be put into an after_hook() function and called in run() after running any of the commands.
• Add bashly add hooks
• Add hooks example (== integration test)
• Update bashly docs (Add Before/After hooks documentation bashly-book#21)