More oil keymaps #132

Keymaps:

• Yank entry under cursor get_cursor_entry
• Yank current oil directory get_current_dir
• Specific sort, i.e. most recently modified files at top
• Open prompt to type in external command which is then run with the last argument being the current oil entry

Might also redo some existing ones, I don't know how I feel about local leader in these cases.

Perhaps use ctrl instead of g as a prefix, as toggling hidden files is a bit awkward at the moment.