Feature: add graphql pagination support

[yanyongyu]

resolve #112

Example usage

from githubkit import GitHub

g = GitHub("your token here")

query = """
query($owner:String!, $repo: String!, $cursor: String) {
  repository(owner:$owner, name:$repo) {
    issues(first: 10, after: $cursor) {
      nodes {
        number
      }
      pageInfo {
        hasNextPage
        endCursor
      }
    }
  }
}
"""

for result in g.graphql.paginate(query, {"owner": "owner", "repo": "repo"}):
    for issue in result["repository"]["issues"]["nodes"]:
        print(issue)

[codecov]

Codecov Report

Attention: Patch coverage is 91.81818% with 9 lines in your changes missing coverage. Please review.

Project coverage is 17.64%. Comparing base (a90820a) to head (0e080c6).

Files	Patch %	Lines
githubkit/graphql/__init__.py	89.18%	4 Missing ⚠️
githubkit/graphql/paginator.py	93.44%	4 Missing ⚠️
githubkit/exception.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   17.61%   17.64%   +0.03%     
==========================================
  Files        4791     4792       +1     
  Lines      222451   222542      +91     
==========================================
+ Hits        39191    39276      +85     
- Misses     183260   183266       +6     

Flag	Coverage Δ
unittests	17.64% <91.81%> (+0.03%)	⬆️

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