patches for #60 and #56

[152334H]

don't mind the dirty commit history, the changes are

1. strip \u200B (aka Zero Width Spaces) from problem statements
2. add Error::PremiumError for queries to premium problems with non-premium accounts (Error::CookieError returns when LEETCODE_SESSION isn't valid in general)

example:

$ cargo run p 1214
[1214] Two Sum BSTs is on the run...
error: Your leetcode account lacks a premium subscription, which the given problem requires.
If this looks like a mistake, please open a new issue at: https://github.com/clearloop/leetcode-cli/

[clearloop]

Thanks!