Fix Escape mapping in insert mode

Fixes https://github.com/cohama/lexima.vim/issues/10.

The hack here is to setup an `<Esc><Esc>` mapping, which would cause Vim
to run into a timeout for cursor keys etc, and then use a `<nowait>`
mapping for just `<Esc>`

This removes the `g:lexima_no_map_to_escape` setting and adds a new one
to configure the mapping to be used: `g:lexima_map_escape`.