Add --header option to import CLI

[jtnelson]

Add an option for the user to specify the header on the command line. This will help in cases when the header is not provided in the file but the user doesn't want to write custom code to provide the header.

In the implementation, ImportCli should set the header in each importer. Each importer should also check the first line of input against the header that was provided. If the first line of input is identical to the header, that line should be skipped during import. This is meant to help in cases where multiple files are being imported, but only one of them has the header. In that case, the user can simply specify the header on the command line.

[jtnelson]

This is also a good option when tailing a log file, the header can be specific on the command line so the data gets routed to the appropriate keys and there is no need to create a custom importer