Multi-folder mode

Just started using this plugin, and I generally like it.  Here are some things I added to make it more useful to me:

- Added multi-folder mode in settings file allowing a user to quickly add more than just one folder from the history list
- Refer to items in the menu as the "history"  (this can be reverted if desired; history seemed to make more sense to me)
- History limit is now configurable but defaulted to 66 as was originally
- Added a command in the menu to clear all folders (available in multi-folder mode only)
- Ensure if you are using this plugin with a project, and the project has relative paths, that the history items are converted to absolute paths
- On start up, audit the folder list and remove dead links

Feel free to exclude any or all things if they take the plugin in an undesirable direction.