Allow simple search terms on source pages

[GoogleCodeExporter]

The new built-in Markdown rendering is awesome :)

In order to make it even more awesome for doing high-level code documentation, 
I'm proposing that gitiles pages that render source files also accept a new GET 
parameter `s=TERM` which will scroll the page down to the first instance of 
TERM.

A possible implementation:
  * in BlobSoyData#prettify, track for the Nth TERM. On the first hit, insert an anchor #TERM_N
  * in the javascript, if no #anchor exists in the URL and the s=TERM is present, set the anchor to #TERM when the document becomes ready.

Original issue reported on code.google.com by iannucci@chromium.org on 20 Jul 2015 at 5:24