feat(mr create): add --reviewer parameter

[pataar]

Description
This adds the --reviewer parameter to mr create. Which allows reviewers to be set on create. See: https://docs.gitlab.com/ee/api/merge_requests.html#create-mr

Related Issue
How Has This Been Tested?
I've used the binary built from this fork against a private Gitlab instance. Worked as expected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)

[codecov]

Codecov Report

Merging #854 (5dcdf87) into trunk (5d497fd) will decrease coverage by 0.03%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##            trunk     #854      +/-   ##
==========================================
- Coverage   59.35%   59.32%   -0.04%     
==========================================
  Files          93       93              
  Lines        7374     7380       +6     
==========================================
+ Hits         4377     4378       +1     
- Misses       2614     2618       +4     
- Partials      383      384       +1     

Impacted Files	Coverage Δ
commands/mr/create/mr_create.go	33.24% <16.66%> (-0.26%)	⬇️

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[zemzale]

This won't show up with the interactive prompt, but this is better than not having the ability to set the reviewer, so I am gonna merge this(I have wanted to do this myself, but never got around it 😄)

Thanks for this @pataar