Improved opening file in existing or new tab.

The `tabnew` command always opens a new tab, it is easier to let vim check if the file is open and reuse the tab or open a new tab by using the `tab drop <path>` command.

Maybe this could be applied to opening files in buffers also, using the `drop <path>` instead of the `edit` command. I think this could avoid some autocommands to be called unnecessarily.

Reference:
http://vim.wikia.com/wiki/Edit_a_file_or_jump_to_it_if_already_open

Possibly related issues:
https://github.com/davidhalter/jedi-vim/issues/65
https://github.com/davidhalter/jedi-vim/issues/87