Rename "ui" to "cli"

The current interface more specifically fits the term cli, so I renamed
the class and all places it's mentioned.
Inital Documentation
Change terminology from "goal" to "target"
Add silent command line interface

In order to use this program as a backend, add the silent mode, which
takes two arguments: "-c" to indicate the mode, and "[target
[collection]]".

The cmd_test.clj tests the command line interface, the core_test.clj
tests the new main function.
Start developing a function to create a 6numbers problem

Very unfinished - nothing depends on these yet.
Formatting