Add help section extraction support

[nkakouros]

Due diligence

• [x] I am familiar with the "Reporting issues" section of CONTRIBUTING.md
• [x] I have searched the existing issues and haven't found one that matches what I'm reporting or requesting now.
• [x] If this is a bug report, I have searched the Bash changelog for information that may explain or provide clues to the behavior I'm observing and reference it in the body of the report.

Description

It would be handy to have a function that returns the Usage or Options sections from the help text. This could be used if a command is called without its expected arguments or with wrong options. Then the usage or options doc could be returned.