tmux: Set base-index before sourcing powerline

Otherwise it gets ignored
vim: remove myusuf3/numbers.vim

As of vim 7.4, it is now possible to use a kind of "hybrid" mode by
setting both number and relativenumber. You will get relative numbers on
all lines except for the current line, which shows the absolute number.

While not entirely the same as what numbers.vim did, it suits me better.