Improved preview and added support for settings file

Significantly improved the preview functionality.
Added feature to remove any non-existent files from the history when previewed or opened.
Added use of settings file to retrieve user preferences:
* history_file: Path to store the history entries in (relative to the sublime packages path)
* global_max_entries: Maximum number of history entries we should keep (older entries truncated)
* project_max_entries: Maximum number of history entries we should keep (older entries truncated)
* use_saved_position: If we should try to use the saved position of the file or blindly use the "new_tab_position" setting
* new_tab_position: Which position to open a file at when the saved index in no longer valid ("next", "first", "last")
* show_file_preview: Should we show a preview of the history entries
* remove_non_existent_files_on_preview: Remove any non-existent files from the history (when previewed or opened)
* debug: Print out the debug text?