Added auto-trigger support for single-key commands

With these changes, you can disable `smartparens-mode` and handle parentheses and quotation matching entirely through yasnippet. I added a new key `trigger` to the snippet definition file to support this behavior. Here is how you would define a snippet to automatically match parentheses.

    # -*- mode: snippet; require-final-newline: nil -*-
    # name: (
    # key: (
    # binding: direct-keybinding
    # trigger: auto
    # --
    ($1)$0