Consolidate options to copy, move, symlink, and hardlink files

Excuse me for not following exactly the Issue template, but I hope it's OK since It's just a question I'd would like to open for discussion:
I'm rather confused with some of the configuration options, especially in the `import` section - Why is there both the options `import.copy` and `import.move`? Wouldn't it be simpler to use one option only? And here's another example - According [this page in the documentation](http://beets.readthedocs.io/en/latest/reference/config.html#link), It seems that `import.link` is boolean, OK but it conflicts with `import.hardlink`. Would it be possible to have only `import.link` which could accept the value `hard` as well?

According to the documentation, it seems that `import.copy` and `import.move` may conflict and `import.link` and `import.hardlink` may conflict as well between them and with the previous options as well! I think it will be wiser to have 1 option named something like: `import.operation` and it's optional values should be `copy`, `move`, `link` and `hardlink`.

What do you think? Is it worth making a PR for this?