Proxing all small web through /x/ might not be desirable. In this commit I am adding a new argument, -p, that allows proxing all external requests using a third party proxy, like Portal (https://portal.mozz.us).
By proding a value for this parameter any request made to `/x/` whose host does not match the root host will be rejected.
By default, kineto loads a stylesheet given to -s from disk and places
it inline with the HTML in a <style>...</style> block. This patch adds
a -e flag to load a stylesheet externally. When the -e flag is passed
with a URI (relative or absolute), the given link is placed in the href
of a <link rel="stylesheet"...> tag. This helps facilitate caching
which can *significantly* reduce request overhead, particularly when the
stylesheet is large (>= the size of the page content).
The given URI is not validated, and if it is invalid the browser will
404 when requesting it and the page will have no style.
