Change cursor to block when Vim starts up

Old:
let &t_ti = enable_focus_reporting . save_screen
New:
let &t_ti = cursor_to_block . enable_focus_reporting . save_screen

Also changed doc file to reflect this:
|t_ti|  Vim startup: change cursor to block, enable focus reporting, save the screen.

Before this change, if someone uses a cursor other than block in their shell, vim will start up with that cursor shape even though they're in command mode.  I personally had this problem because I use the bar for insert mode in zsh vi-mode (ironically mirroring what this plugin does).  This commit fixes that by including the cursor block escape sequence in the startup termcap.