add option to specify template

This allows to pass a custome template like this:

    cat ./changelog.hbs
        ## {{header}} 

        {{#issues}} * @{{user.login}}: {{title}} (#{{number}})
        {{/issues}}

    gh-changelog -t ./changelog.hbs


and have access to every value from http://developer.github.com/v3/issues/#list-issues-for-a-repository