Use a more standardized project layout

In #2, mrngm made the point that we should move to a more
standardized project structure. This commit does exactly that.
The new project structure is based on the repository listed at
<https://github.com/golang-standards/project-layout>.

Fixes #2.

assets/text/index.txt

@@ -0,0 +1,10 @@
+#RU URL SHORTENER
+=================
+
+Based on https://0x0.st/, this site allows you to easily shorten URLs using
+the command line.
+
+## USAGE
+
+    # Shorten a URL
+    curl -F'shorten=http://example.com/some/long/url' https://hashru.link