Watch mode for mismiy

The mismiy command now has a --watch option that causes it to monitor the posts and partials directories for changes, and re-renders the site when files are modified, added, or deleted. The idea is you run

mismiy posts --watch

in one window, and the web server in another, and then as you edit the posts or the templates, the site is kept up to date. You still need to refresh the page in the browser to see changes.