Make skydb load examples/home.sky by default.

I had to also register for the text/plain mime-type
which turned out to be harder than expected.
Mostly due to my confusion with mojo_shell
using last-argument-wins argument parsing.

R=ianh@google.com

Review URL: https://codereview.chromium.org/672363002

examples/home.sky

 #!mojo mojo:sky
+<sky>
 <style>
  h1 { font-size: 2em; margin: 1em; }
  p { margin: 0.5em 1em; color: #bcd8f5; font-weight: 900; }
 </style>
 <h1>about:blank</h1>
 <p>Welcome to Sky!</p>
+</sky>