Why:

* Some analyzers need to write the custom CA to a file that already has
  other certificates within it.
* The analyzers have different locations where they need the CA
  certificate to be written.

This change addresses the need by:

* Update bundle write function to append the certificate if the file
  already exists.
* Update the Import function to allow the location of the CA certificate
  bundle file to be specified.
* Update the command.Config to pass through cacert.ImportOptions

Side effects:

* The change the cacert.Import function signature is a breaking change
  and will require work in each analyzer that doesn't use the command
  package and calls that function directly.