update preview when the user types

We don't have any delay in between updates (because i'm scared of
threading), which has a few problem:

1. probably really sluggish on slow systems
2. probably slow for readmes with images (need to test)
3. flickers (the phantoms are updated too quickly, so sometimes it
doesn't replace the old one smoothly)

BUG: the preview doesn't load when we preview the markdown file

test.md

@@ -0,0 +1,8 @@
+# hello world
+
+This is a *test*.
+
+I'm not sure that it **actually** going to work, but it seems nicer than the [previous version][prev]
+
+
+[prev]: https://github.com/math2001/MarkdownLivePreview/tree/d4c477749ce7e77b8e9fc85464a2488f003c45bc