Introduces magit-read-directory-name and magit-read-file-name, these all...

...ow the user to set custom variables to control which function is used to prompt for directories and filenames, such as setting magit-read-directory-name to ido-read-directory-name.

The only instance I couldn't figure out how to get working with ```magit-read-directory-name``` was the relative argument ```=r``` for logging, so I left it as ```read-directory-name```.