Adding phantomjs-url-template option

It would be nice to have an option phantomjs-url-template, that allows one to use a template instead of a url base (in case we have to upload to a repository system that doesn't provide the same file format as phantomjs does).

Here's the snippet from the readme:


    phantomjs-url-template
        If phantomjs-url and phantomjs-url-template are
        not specified, you can set a template which will populate various
        variables. The variables should be wrapped in {}, and the
        following values are supported:

        * arch: the architecture. x86_64 or i686
        * phantom_platform: the platform, following the format dictated by the standard phantomjs url (e.g. linux, macosx)
        * phantom_extension: the extension, as specified by the format dictated by the standard phantomjs url (e.g. tar.bz2, zip)
        * platform: the platform, as specified by sys.platform (e.g. linux, darwin)
        * version: the version of phantomjs

        The default template is:

            https://bitbucket.org/ariya/phantomjs/downloads/phantomjs-{version}-{phantom_platform}.{phantom_extension}