clear screen as part of the command

This is a little faster and it also deals with race conditions when
`&t_te` and `t_ti` are doing things like enabling/disabling focus
tracking with vitality.vim.
Merge pull request #33 from aaronjensen/clear-as-prefix

clear screen as part of the command
Add quickfix option
add silent to map
Re-order prefix determination (zeus last)