Working for repos with more than 10 pull requests

Right now, the extension works only for public repos that have 10 or less pull requests. One of the reasons for this is the GitHub's rate limit of 60 requests per hour. The user can quickly run out of "free" requests if he/she decides to check multiple files within different repos.

The only way to increase the rate limit is by using a personal token. The personal token can be pasted in an options page related to the extension. Much like issue #13 . This puts the limit to 5000 requests per hour and also gives the possibility of using GraphQL (v4 of the API). The later can be used to combine multiple REST calls giving the flexibility to check a big number of PRs at once.