Replace the daemon invocation with a more idempotent equivalent.

This avoids invoking the daemon if it's already running, with helps
avoid opening a bunch of spurious browser tabs.  It uses pgrep which
should come with most Unixes (confirmed for OS X, CentOS, Ubuntu)
Add instant_markdown_gc and instant_markdown_finally_kill_daemon config options.

These options control how the underlying instant-markdown-d nodejs
daemon is manipulated by the plugin.  The daemon can be
opportunistically culled (the existing behavior, and is the default
still) or allowed to persist.  The daemon controls the browser tabs so
these options do affect the UX.