refactor(CLI): Make check command more modular

[adleong]

We slightly refactor the linkerd check command into more modular and reusable pieces. In particular, we split the configureAndRunChecks function into two pieces: ConfigureChecks and RunChecks, making both of them public. This allows them to be invoked individually and to potentially modify the HealthChecker in between.

To further support that use-case, we add a InsertCategoriesBefore function to the HealthChecker that allows inserting checks at particular locations in the order rather than only being able to append them to the end.

[alpeb]

What do you think about making the checkOptions struct public, instead of introducing this public var? The newCmdCheck() function would need to become public as well, and refactored to receive a pointer to that struct, instantiated by the caller. Similar to what was done with NewCmdInject() to provide more flexibility there.

[adleong]

unfortunately, making newCmdCheck public wouldn't be sufficient for adding new checks to the Healthchecker because the cmd that newCmdCheck returns always uses the same list of checks.