support URL shortener

[philippb]

Markdown links get rendered in their text representation as Linkname (link). When using full URLs this will break the layout of the text.

URL shortener config

ks-email-parser should support URL shortener for text rendering.

The shortener should be configurable through the config file url-shortener or command line param --url-shortener.
The value for shortener us a URL that gets called with a GET request and a placeholder {{url}} for the URL that gets shorten. e.g.

url-shortener: http://www.getkeepsafe.com/shortener/{{url}}

The shortened URL is return in the body of the request. This will be compatible with our own URL shortener and bit.ly.

Using url shortener

When rendering the text representation of emails, all URLs should be shortened when a URL shortener is defined in the config or through command line.