Be able to reset to level given name

When I run `githug reset` w/ an argument, it was unintuitive to me that a file path was expected and the level in that file would be loaded. As I was doing the Git exercises, there were points were I wanted to go back to the previous level and redo it again. I think the argument for `githug reset` should be the name of the level the user wants to reset to. 

This means there should also be a command to list the names of all the levels and a new command to actually load a level in a file given the path of the file. 

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The tests in `cli_spec.rb` and `level_spec.rb` need to be updated. I was going through them and I don't have enough experience with RSpec just yet to be able to update them. I'd love to look over the updates when they're made though however. 

I also think the formatting of the output of `$ githug levels` should be tweaked slightly.