Remove '?' prefix in FeedOption::build_url

[Jaltaire]

Repeated inclusion of "?" to identify the query string will cause any parameters to which "?" is appended to be treated as "param?". This leads to the parameter being ignored by the Reddit API as it cannot be parsed correctly.
For example, when calling Subreddit::new("ipad").latest(1, Some(FeedOption::new()), the URL will be formatted as "https://www.reddit.com/r/Watchexchange/new.json?limit=1?", and limit=1 is ignored.

For now, let's remove the "?" appending in FeedOption::build_url and instead include it in every URL passed to build_url.

Ultimately, we should probably rethink how we're specifying options. Current design, for example, allows limit to be specified twice in many places. The second limit specified in the URL will always be the one used (overriding the first).

Likewise, the splitting of options between FeedOptions and as function parameters makes for a confusing API. If FeedOptions are truly applicable everywhere, then we should remove redundant provisions of limit. If certain options (e.g., before and after) are only applicable in certain places, then perhaps we can forgo FeedOptions and provide the appropriate parameters for each function corresponding to a REST API call.

[Lurchfresser]

Noticed the same issue, your pull request seems to fix the issue, even though I didnt test it. Also agreeing on the FeedOptions take.

[beanpuppy]

Agreed @Jaltaire. However, I'm not particularly intertested in maintaining this library anymore (I've wanted to make an issue looking for new maintainers for a while) so I doubt I'll put much work to it, but I'd be happy to merge any new design you come up with if you wanted.