Remove similar code for command-line argument handling

[aeslaughter]

Reason

The CommandLine::initForMultiApp method and the Parser::hitCLIFilter both have logic that is similar for separating MOOSE hit syntax from the supplied command-line arguments.

Design

These two methods should share a common code path or perhaps the use of one or the other can be removed. The reason for the current setup (see #13433) is to allow each MultiApp to have a unique CommandLine object.

Impact

The main goal is to remove duplicate logic, any impact should be internal and not influence operation.