Accept single and multiple files per method invocation

There were methods which accepts only a single file per method invocation and there were methods which accepts multiple files (as arguments). I added functionality which makes it possible to use multiple files per method invocation. One does not need to put those file names in an array. I use the splat operator to make this possible. The only "drawback" is, that it is not possible to set default values when the splat operator is in use. I work around this by checking via `#nil?`.

```ruby
touch_file 'file1', 'file2'
```