Add support for atcoder

[cdbalaji]

Add atcoder support. Suggested prefix: at:

[ankur-kayal]

Research work done:

1. Atcoder has no APIs to fetch problems but we can scrape all the problem data ( sample implementation: https://github.com/kenkoooo/AtCoderProblems/tree/master/atcoder-problems-backend). The only caveat is that atcoder problems don't have problem tags which is the basis of the codedrills recommender).
2. Incorrect solution to the above problem (although it might work till some extent). Atcoder has official solution to the problems, so we can scrape the solution title and scrape the solution heading like bfs approach would give the tag bfs. Might not work for most cases.

[ashishpawar517]

Is there way we can fetch the user submissions Atleast?

[ankur-kayal]

@ashishpawar517 yes we can get user submissions but how will that help? We will not be able to recommend problems from atcoder to the user.

[ashishpawar517]

Actually there's one website called as atcoder tags that has some number of problems with tags, we use that for recommendations. Actually thing that I wanted to see was how can we add other cp sites submissions in codedrills like getting entire cp tracking from all the websites to one platform

[ashishpawar517]

any updates to this @ankur-kayal