CLI: Create ast_cli_completion_add function.

Some completion generators are very inefficent due to the way CLI
requests matches one at a time.  ast_cli_completion_add can be called
multiple times during one invokation of a CLI generator to add all
results without having to reinitialize the search state for each match.

Change-Id: I73d26d270bbbe1e3e6390799cfc1b639e39cceec