Add Push event for GitHub connector

[ajitdsa]

Description

Adding an event that triggers when a user pushes commits to a branch in Github.

Status

READY

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update
• Documentation (fix or adds documentation)

How Has This Been Tested?

Added a test in test_connector_github.py for this functionality. Also tested manually with my own github account.

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #1821 (0fe08ab) into master (c6643da) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1821   +/-   ##
=======================================
  Coverage   99.30%   99.30%           
=======================================
  Files          80       80           
  Lines        4762     4772   +10     
=======================================
+ Hits         4729     4739   +10     
  Misses         33       33           

Impacted Files	Coverage Δ
opsdroid/connector/github/__init__.py	100.00% <100.00%> (ø)
opsdroid/connector/github/events.py	100.00% <100.00%> (ø)

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