Use TERM 'screen-256color' for tmux & ncurses compatibility

Inside tmux sesssions, ncurses views wouldn't draw properly.
Setting TERM as 'screen-256color' will solve the issue.
Vim: Update fugitive key mappings

After updating vim-fugitive, the key mappings has to be updated.

- <leader>gd : git diff, with vertical split
- <leader>gD : git diff --staged (diff with HEAD)
- <leader>gb : git blame -w (ignore whitespace), with small width
  (press A/C/D to resize to end of author/commit/date column)