Feature:(issue_1927) Add ability for before handler to return new context

[dearchap]

What type of PR is this?

(REQUIRED)

• feature

What this PR does / why we need it:

(REQUIRED)

Which issue(s) this PR fixes:

(REQUIRED)

Fixes #1927

Special notes for your reviewer:

(fill-in or delete this section)

Testing

(fill-in or delete this section)

go test -run=TestCommand_Run_BeforeReturnNewContext

Release Notes

(REQUIRED)

The Before handler for a command can optionally return a new context.Context which will then be propagated to the Action and After handlers. 

[meatballhat]

I like it! I had the one question and it looks like the docs tests failures are related to the change in func signature.